Hmmmm.... this is a tough one and can be very divisive.
Girls can charge what they like.
On the whole, I think most girls know their local market and where they want to position themselves within that.
A more expensive girl might have some quiet periods, where as a less expensive girl may be busy all the while, and that can have an influence on how a girl chooses to position herself in the market.
But I like the phrase "What the market will bear"... it basically means that if you charge too much, you won't get any business. In escorting, though, like in most things, "too much" is very subjective.
It would seem, though, that southern-man69 has made a fundamental error in thinking that a girl who charges more should offer more services. No, in fact the opposite is often true. Girls will often target a niche audience and specialise in catering exclusively to them.
I don't do web cam or phone or SMS or sales. I easily could, but I choose to do escorting only. Why? Because there are loads of t-girls doing webcam and phone and SMS and sales... and I wanted to mark myself out as being different to them. I am not a Jack of All Trades... I am a Master of One - the real life, one on one genuine t-Girlfriend Experience.
In fact, while my profile has changed little over the past couple of years, one thing I have done is take stuff off it. I offer less than I did a couple of years ago... but that doesn't mean guys are not getting good value for money, it means that they are getting a honed, tailored service in something very specific.
Each girl has her own way of working and works in a way in which she feels comfortable. And that is as it should be. It is for guys, then, to look at the girls and what they have on offer and choose who they want to see. Different guys have different desires and different expectations. And there is such a wealth and variety of girls out there that all you lucky guys can be catered for! How amazing is that?!?!