... that offers more than massages. It's been a long time since I did that, but when I did the massage was around $60-65 an hour at an AMP, and the customary tip for a HJ at that time was $40. For a little more you might be able to get a BJ, with the masseuse still clothed, but I never went down that path at an AMP.
As swim said, it's going to cost you more than $50.
I have a CFNM fetish (Clothed Female Naked Male) but no escort I've ever talked to has heard of it and the few I asked thought it was weird; in fact, only the sexy maids I've asked have been comfortable with it (though I've never actually tried it).
What do you think is a fair offer for it? 50?
I seriously doubt that very many high quality providers on this site would do anything for just $50. I have seen CFNM clips on the net, but never heard of anyone doing it in a session with a provider.
Swim
... that offers more than massages. It's been a long time since I did that, but when I did the massage was around $60-65 an hour at an AMP, and the customary tip for a HJ at that time was $40. For a little more you might be able to get a BJ, with the masseuse still clothed, but I never went down that path at an AMP.
As swim said, it's going to cost you more than $50.
The total might come out a little more in some areas, but I think you are in the right general neighborhood. I pay a total of $130 for this service at a local place, but I'm a generous tipper, probably $20 over the market rate (any thriftiness I had just 2 years ago left me when I started engaging in this ridiculously expensive hobby lol!).
You don't even have to mention anything before the date. Once you're approved and are in the room with her, just tell her to keep her clothes on since that's your fantasy. I don't think any provider would be opposed to keeping their clothes on, unless you plan on cumming on her clothes.
What do you think is a fair offer for it? 50?
I think he meant he wants to pay $50 total for this fetish session.
Personally, I will spend the same amount of time and effort in preparation for a date (and set aside the same amount of time) that I expect to be FS as for one that is not.... so my rate remains the same.
...at least as far as I'm concerned. I do sessions like this all the time.
I do charge less, but the session is charged as a bdsm session and costs far more than what you've suggested it should cost.
Please keep in mind that regardless if I am naked or not, you're still requesting a specific skill and fetish that is illegal to perform in every state except one. 
just in certain counties.
Or what you think it should cost. Time to a lady is time, you should be comfortable with her rate. I recently met a gentleman that had a very specific request, and once he was screened, he let me know so I would be comfortable with what he was asking. I had him send me video clips to get a better idea of the type of experience he was seeking and if I would be able to visualize and provide what he liked, and when the appointment came, him throughly enjoyed himself. This has not been the first time a request has come up for a fetish that does or does not involve sex, but all gentlemen that I have encountered, pay the price of admission for what they want. Finding an open minded and comfortable provider for your fetish may be more of a challenge, but we are willing and out there! Good luck!