I guess this topic keeps on coming back.
SPs (independents anyway) are independent contractors, they tell you what their rate is and you either agree to it or you don't see them. Whatever a "tip" is in that scenario, it is built into her rate, so tipping is not necessary. If she wants more, she will increase her rate. In fact, if she is regarded as professional, tipping her is actually demeaning. You don't tip your doctor, or your dentist, your plumber, or your financial advisor - no one would even think of doing that. I think it is somewhat different with agency girls since they are for all practical purposes employees, so there might be a better argument for tips in their case.
Normally the general rule is that if the person providing the service is being paid minimum wage and depends on tips to make up the balance for a living wage, then you tip. If they are being paid a living wage, or are independent contractors, you don't tip.
As far as an SPs actual rate is concerned, I don't think it can really be measured against the level of service provided by other SPs. There will be generally accepted guidelines in the market based on what most other SPs are charging, since she will want to get the most money she can for the service, but at the same time not have a fee so high that no one will see her. Within those constraints she will set a fee she considers reasonable. Plus, there will be an element of filtering involved - if an SPs fee is on the higher end, she is less likely to see "undesirable" clients (ie the bottomfeeders).