what vaccinations do kittens absolutely need?

IMO, a single rabies and a single FVRCP are necessary at around 5-6 months. Some people think rabies is unnecessary for indoor cats but I give it in case my cats get outside. Some people prefer to give 2-3 rounds of FVRCP. Its basically what your comfortable with and what you think is best for your cat. Most agree that other vaccines (like FIP and FeLV) are completely unnecessary for indoor cats.
If I remember correctly my vet charges about $20 per vaccine. Different vets will have different prices.

Kittens that are to be kept indoors only need two - they need a series of two or three (depending on age) FVRCP vaccines, and one rabies vaccine. No others are needed nor recommended for an indoor cat. The costs are the vet exam, and the vaccines. Prices vary you can call a local vet office.