Escorts and mistresse are completely different... it is misleading to think of a mistress as a long-term escort.
Sometimes a man falls for a woman, but isn't willing to leave his wife (or doesn't want to... its possible to love more than one person) or its something where a man and woman have a great romantic chemistry, even love, but they know it could never work as a marriage. this (in my experience) is the origin of the mistress arrangement.
I have had mistresses twice, once 8 years ago, and once about two years ago. Both lasted about a year, and in the case of the former it coincided with me having a finace at the time, and in the latter I had (and still have) a wife. These were cases where I met these girls, and while I would never leave my wife, I HAD to possess these girls... so beautiful, so intelligent and witty, sexy, but also much my junior (the first case I was 29 and she was 19, the second case I was 35 and she was 20). I supported them completely, sent the first one to school and got the other a couple of gallery showings. So, its not about "sex" per se, but about kind of taking care of someone you *might* have loved as a young man. An escort, conversely, is primarily all about recreational sex; hence our term, "provider".
Obviously, this is my experience, and I'm offering it anecdotaly, not as a definition or theory.
Just last Thursday I met a flawlessly beautiful Russian artist (20yo) who may be my next "mistress" - she is so incredible, and I want to see her succeed and have a happy life, so I'll help her connect to the art world here, go shopping with her, etc. Maybe it will happen, maybe not... if so, it lasts maybe a year, we each learn things, its good all around if its done right. Expense isn't necessarily that extreme; depends on the situation; really, its a kind of relationship.
Men by their nature want to take care of something; a mistress satisfies this. In a way, there is almost a mentor/pupil or father/daughter quality to it.
So escort vs. mistress? Apple vs. Orange, sometimes you like both, and one needn't exclude the other.
O.