This is an answer to the question almost posed in the title "What is the cost of hiring a porter for Everest base camp?" I won't talk about the money you would need to shell out, but I will argue that the costs are higher than you think. The main points are:
- There are more effective ways to use your money to improve Nepali lives
- There are possible ethical problems in hiring a porter
- Going without a porter is not only possible, but may be more enjoyable
- Altitude kills, and porters (but more so guides) push you to go to higher altitudes faster
First, it's great that you're thinking about the moral/ethical issue of visiting a poor country as a person who is -- by Nepali standards at least -- very rich. This is a country that will surely delight you, and its people are almost universally warm and welcoming in my experience. They've been through some tough times in recent years, and they could certainly use some more money. You will already be adding money to the local economy on your trek by paying to stay in lodges and eat their food. I think it's great that you want to give more money, and I encourage everyone to do so. I wouldn't say it's morally obligatory, but it's really nice.
But then the question is: where should you give money? Hiring a porter is okay. As @grm pointed out, porter agencies frequently act as middlemen taking a big slice out of their pay, so you're mostly giving money to relatively rich Kathmanduites. You might be able to deal directly, but be aware that there are reports of crooked porters who will just take your stuff, so you need to be careful about payment. It seems that the best way is to ask for individual recommendations from people who have gone previously. And with or without an agency, you need to be sure that the person who actually walks with you is equipped to go to the places you are going (shoes and coat appropriate for high altitudes), and is being paid a decent rate. These are not automatic.
Ultimately, I think it's clear that if you take the money you save by not getting a porter, and spend it elsewhere, you can do more good. Look at Educate the Children Nepal as just one example of many great charities in Nepal. (I'm not affiliated, but I'm a big fan.) A huge fraction of the money you donate to them goes directly to empowering women, educating children, and developing agriculture. The money you give to a porter (or the agency arranging a porter for you) may go to similar things, but probably not as efficiently. I would guess that it's more likely your money will go to enforcing patriarchy and increasing class/caste divisions.
On a practical level, it's entirely possible to go without a porter. I've done the long version of the Everest Base Camp hike twice now, both times without a porter. (The long version involves leaving from Shivalaya or thereabouts, and I highly recommend it over flying to Lukla.) With or without a porter, you will have to adjust physically to the demanding work of moving yourself up and down the mountains. Unless you pack a truly unreasonable amount of gear, it won't make much difference if you've got 25 pounds on your back or just 5. So if you can do one, you can do the other. In fact, if you're flying in to Lukla, most of the time you'll be altitude limited, so you simply won't be able to take long hikes safely, and you'll be looking for things to do. Moreover, I consistently find that people who do it on their own enjoy it more. I don't know if it's correlation or causation, but people on their own seem to be more interested in making friends -- both tourist and local.
Finally, porters -- though to a greater extent, guides -- can actually endanger you. They will want to finish this job as soon as possible, so they will push you to keep going to higher altitudes without taking the necessary rest. I've personally witnessed a dozen bodies being taken out of the Everest region. These were not climbers, just ordinary trekkers who went too high too fast, and refused to pay attention to the symptoms. I've also witnessed even more people being evacuated alive but crippled by the altitude. Their guides frequently get a free helicopter ride back to Lukla or Kathmandu, and the porters get to high-tail it down the mountain -- both getting to look for another job. I've even overheard several conversations in which some of those people who were later evacuated said they wanted to rest and acclimatize, but their porters or guides pushed them to keep going. And they kept going, which is the only reason they needed to be evacuated. There's essentially no risk of altitude sickness when hiking to base camp if you're willing to stop and rest until you feel perfectly healthy. But that may mean taking a few more days, which is not what guides and porters want to do. I'm not saying that most porters or guides want their clients to need evacuation, but I believe that some do, and there is definitely moral hazard, where their interests are opposite to yours.
So don't feel like you need to hire a porter. If you're old, disabled, or are traveling with small children, then go for it; it could be a great way to increase your mobility. But if you're a healthy adult, there's just no good reason to hire a porter on a safe and simple hike like the EBC (or other popular routes around Nepal).
 Incidentally, this increased enjoyment goes at least double for people who walk in versus fly straight to Lukla. The walkers have had plenty of time to acclimatize, get in shape, enjoy a quieter route with its varied scenery, and get to know the people. Those who fly in are often physically miserable at least as far as Tengboche, are crushed by the crowds, and only encounter Nepalis who are just there to extract money from tourists. Not that I blame them for making money, but it's nice to just encounter a farmer tending his animals, or children playing in a field. The vibe changes above Lukla, and not in a good way.
 Porters don't usually speak English very well, and have a more deferential role typically. So guides really are the main problem in this respect. If you only get a porter, it'll be less of an issue. But every member you add to your party will be one more voice in your internal monologue telling you that you need to soldier on up the hill. That little voice in your head is what actually kills in most deaths due to altitude. Just be scrupulously honest with yourself, and brutally strict with yourself if the time comes to rest or turn back down the mountain.
 Other ways to avoid this while still lightening your load for the hard part include: (1) hiring a porter just up to Namche; (2) shipping heavier items (extra warm clothes, bigger boots) to Namche; (3) buying things in Namche. As I mentioned above, you'll be limited to short-ish days above Tengboche because longer walks will take you too high (though day hikes are good, but you'll leave your main pack at the lodge). So you'll be perfectly capable of taking a heavier pack up there. You'll also want even warmer clothes than you had at lower elevations. And the walk down is a breeze.