Under no circumstances should you pay a deposit. These escorts are only in it for the money, if they get money from you (the deposit) for doing nothing, it is very likely that they are not going ahead with the booking and will scam you.
Even if you met them before, don't pay a deposit. As they can still scam you.
If they still insist on a deposit, book elsewhere. Plenty of other guys that don't require a deposit.