A few charge a bit more than that but I don't see those women. My rule is I don't send them unless I can verify the lady in question will handle deposits with care and professionalism, and I can only do that by her having a lengthy reputation that I can research. My max would be about 30% but I think I have only been asked for 25% at the most.  
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Since you are paying 20% now, 25% doesn't seem to me to be prohibitive but I cant make that call for you. You have to play in a manner that makes you comfortable.

I get the impression that SOME ask for a deposit to cover some or all of the non-refundable out of pocket expenses. E.g., if there is a hotel expense for an incall, she might ask for some (1/1, 1/2, or 1/3 for her appointments there that day) or all (only 1 appointment - yours - that day) of the room expense so as not to be burned by a no-show.  As hotel costs go up and hotels become more rigid regarding late cancellations, the deposit request goes up.  
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For an outcall, there might be a deposit related to travel expenses on the chance that she gets NCNSed upon arrival.  
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On the flip side, a lot of guys protest that they have NEVER been a no-show but have, themselves, been NCNSed upon arrival to an incall.  
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Everyone has other non-refundable expenses: cancelling meetings, leaving work early and other preparation to get to the appointment; hair and nail salon vs barber and nail salon; etc..  
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There have many prior threads and discussions of Business Expenses but I can't find them easily. Here's one:  
http://www.theeroticreview.com/discussion-boards/ter-general-board-12/re-the-biggest-downside-to-not-tipping----1017097
Those are SOME of the factors that will influence the deposit ask.

Spot on with non-refundable hotel expenses. The costs keeps going up on the cost of rooms and the cancellation policy has gotten more strict, often 2 days ahead. I understand some percentage for incall for sure. Outcall also has expenses, but not nearly as much as incall.

Personally, I ask for 25-50% depending on a few factors....

If I am touring, I ask for 50%, because I have very limited time, so if someone should cancel and not pay a cancellation fee, it could be very detrimental!

Nowadays the market is rather saturated and a lot of us at a certain price point are low-volume. Higher deposits make sure our time is covered. Even if it is just one session in a day, planned a couple to several weeks out, it can really mess up our schedule and overall income to deal with a cancellation.  

Always do your research before paying a deposit of course! xoxox

Do what you feel comfortable with.

It’s your decision to make & you know best. I require more because it’s how I operate. I’m verified & bonafide it’s not easy to be tops. 50% is my deposit 💸 not including travel.🧳 Do you.