What is the difficulty level and how much would that cost for Everest Base camp hiking? As for me, I'm pretty fit and do regular sport. Also, please tell me the best time of the year to do that.
A few things to understand about trekking in the Himalayas, especially in re: everest base camp trekking:
While it is true that if you trek from Lukla to Base Camp, you will have ascended and descended more than the 8848 meters that Everest is, this is not cumulative. You will be hiking between mountains, not over them, for the most part. (Where you draw the line between hill and mountain becomes subjective, but you aren't using ropes or anything like that.)
Don't imagine you are on some distant feat of manly endurance- you're going to be walking through several villages, and will most likely be passed by villagers carrying crates of chickens, coke, pringles, and beer, so that when you hit a tea house along the way, you'll have something to buy. The guys doing this will not have the great hiking boots, jackets, or fancy gear you will. Still, remember that you weren't born in this terrain, so they're not judging you for it. I'm a fat, bald middle aged man, and I was able to do it just fine.
Definitely fly into Lulka. It may be the scariest part of the whole trip. :)
You will be gaining elevation much of the time, but should be acclimatizing at about the same rate as you walk (if you walk like me). Take breaks when your guide says too.
This is basically a commodity trek. If you go into Thamel and ask for an Everest trek, they will probably have an advertised price. Back in '98, I was spending about 200 rupees a night for food and lodging, (much more as you get closer), plus the 600 rupee entrance fees. Finally, we gave our guides about 2000 rupees at the end - but be sure to negotiate this in advance
Personally, I would encourage you to do the Jomson trek instead of Everest, and then take a $99 round trip flight out of Tribuhaven to see the mountain. Everest trek has far fewer views of the actual mountain, whereas Jomson has Tatopani (a natural hot springs), amazing monasteries and spectacular views. And, to top it all off, you'll spend some time in Pokhara, which is an amazing sight in and of itself.