Well if you are lucky you can find a herp vet that doesn't require the gecko to be seen in order for a fecal but the chances of that are slim.
When I first started taking mine in, I paid a hefty $250 vet bill for one gecko and he didn't even have parasites! Now I can take the poo in without having to bring the geckos because he got to know us really well, you just need their current weights so if the fecal comes back showing something they can get the dosage right.
I would say between $25-$100 depends on the vets requirements, sometimes it could be even cheaper.
I am not sure if mine will do a fecal without seeing the gecko, but when we brought ours in, the fecal was $20. It seems all vets have different rates.