My opinion, for what it is worth, is to pick the girl you want to see regularally and pay her the listed rate. If she is worth seeing twich a month then she is worth her published fee.
If you do become a regular client with a lady, there is always the chance that she may not watch the clock as much when with you if she is not busy. However, this would always be at the escorts discression on a visit by visit basis.
Bottom line: Just pay the fee as listed and have a good time.