Ooh, piggie dentistry. I daresay there will be many views and experiences shared on this topic. Going on what I've learnt and experienced:
1) There is no need to trim the teeth unless there is a problem with the teeth. Trimming unnecessarily can actually create problems. Ample amounts of hay and some hard foods will help to keep healthy teeth in check.
2) Most vets sedate, but it is quite widely agreed among owners, rodentologists and piggie-savvy vets that an anaesthetic is unnecessary. Guinea pigs have no nerves in their teeth, and it's a whole lot less stressful to wrap them up, use the right tools to hold their mouth open and get treatment over and done with in a matter of minutes, than have them spend all day at a vets and be knocked out just for a few minutes. However, you do need a vet with the skill to handle and wrap the guinea pig securely enough that the pig cannot move while treatment is carried out.
3) As said before, no nerves in the teeth so the trimming of the teeth is not painful. Teeth trimmings can go wrong if the person carrying out the work is inexperienced and untrained, and teeth can be trimmed too short which can create more problems. But on the whole, any pain associated with dental trouble is to do with the teeth cutting into the cheeks or tongue, or an oral infection.
4) Cost depends entirely on whether a GA is used. The CCT guinea pig hospital does dental work for free for members. Independent rodentologists, I can't say for sure but I think most also do dental work for free. Vets will charge for the consultation (usually around £20) and nothing more if dental work is done on a conscious piggie, whereas if a GA is used it goes up a bit more.
