As an experienced cruiser, who tips well, I find cruises to be an extra ordinary value. In fact your cruise might cost a bit less than what you actually paid! You could be refunded part of your port fees if the boat leaves a bit early. However, you will likely spend more than that.
Deferring to other answers I will give my insights.
I prepay my tips and also tend to tip extra when I get good service. To me, the tips, are part of the cost of the cruise. Most of those people work their tail off for you, and that is how they make their money. This past cruise I tipped the wait staff extra, because they were very kind/helpful to my wife's food allergy and our cabin steward because we asked for a couple of extra things and he was great.
We tend to use the ship supplied shore excursions despite being a bit more expensive. We have been late coming back, where they had to hold the boat, at least twice. Had they not held the boat flying to the next port would have been costly, and stressful. It is just not worth it. Also we were robbed by using a non-cruise ship excursion one time when we all the cruise ship excursions were booked. No thank you.
Believe it or not the shopping on ship can be a really good value. I have been considering a GoPro and our recent cruise had a competitive price plus no tax. Buying on ship would have been a better deal then buying in the US.
First time cruisers tend to spend a lot on portraits, but the novelty wears off. We tend to use one of our cruse photos for the next year's Christmas card. Pictures are kind of expensive.
One way a lot of gullible people get sucked in is the free "art auctions". Park West is aboard just about every cruise line and they rob people blind. Don't fall for their marketing tactics. They are experts at getting you emotionally attached to stuff you can buy at Home Goods for $60. They will charge you hundreds for the same art.
The same kind of thing goes on in port. The cruise ship will hold seminars on "ports of call" and offer all kinds of incentives and coupons to visit jewelry shops in port. Most are rip offs. Tanzinite International is a major culprit. Now you can get good deals on luxury watches and such, just do your research ahead of time and understand what a good price is to pay.
Bringing things from home can greatly reduce the price of your cruise such as: sunscreen, aloe, water shoes, snorkel gear, medicines, insulated coffee cup, water bottle, id lanyard, book(s), games, sun hat, sunglasses, shawl (the wifey gets cold easily), backpack, etc...
Really cruises are a great way to have a relaxing vacation. Very little is required of you. This past cruse I had a need to get a small internet package, and that was pretty reasonable.
For the record, I sailed NCL in 2018 and will probably not sail them again. There were many people over served and were very obnoxious. We liked the
Cirque du Soleil show on board, and the fact you can make reservations for the various shows. We did do some specialty dining and found the value good. Shore excursions were reasonable. But with the falling down drunk passengers, and the amount of time spent at account services desk the made the vacation less fun than other brands.