Thank you for taking on this huge responsibility and being responsible and spaying and neutering the kittens before they leave.
There are so many full shelters out there that you often see specials every week for cats being adopted out for as little as $15-$25. Kittens are typically a bit higher.
You want to ask a rehoming fee to prevent people who view these animals as disposable (cheap kitten so throw it in the trailer with their 2 year old who carries it around by the neck) or use kittens for food for their large exotic animals but you also want to keep the cost to a point where you are not excluding good homes.
If you can afford to absorb some of the cost associated with raising and taking care of these kittens (vet bills including altering) then I would list the price as a 'small re-homing fee" and speak to all interested parties by phone.
Ask tons of questions to get a feel for the potential home they will be going to. Is it some young college student that is trying to sneak a kitten into a dorm where the kitten will end up on the street or a kill shelter, someone whose dog killed there last cat and they want to replace the cat but not the dog, or someone that wants a kitten because their last one died of a preventable disease like heartworms or being run over in the street? If the homes are not acceptable like the ones I just mentioned decline the sale but if you find a home that you feel is appropriate, keep the fee reasonable such as $25-$50 as your primary goal is to find a good home.
I guess my answer is to just be generic and list a small homing fee and then get a feel for who you are selling to and if you find a decent home, make the price as reasonable as you can afford.
Posting a good photo showing the kitten in an adorable pose will do wonders in finding a nice home. I know good people that had their mind set on only a calico that could not resist the cute little white kitten playing with the ball in the photo.
Good luck
Edit:
To those suggesting the 'minimum' cost should be expenses +.... I would argue that the minimum cost should be what the poster can afford when trying to find these six kittens a proper home.
I rescued several feral kittens over the last five years and voluntarily absorbed the entire cost of vet appointments (several were sick when found), medications, and altering and them and on top of this, paid $75 each to place in a really terrific no kill shelter after socializing them and waiting until a spot opened up. In addition to the cost to rescue these kittens, I PAID to find a suitable home.
The 'minimum' should be what one can afford when rescuing a cat or kittens to find them proper placement. Not everyone expects every penny back when doing a good deed and those who can afford to absorb some of the cost will likely place the kittens much faster. I cannot imagine how long it would take this poster to place SIX kittens if she adds in all her costs + a base fee which likely will amount to $100-$150 per kitten.