Usually it would be in the ad, or on her site... or both.
If there's a special reason you need to ask - i.e. the rates on her site are specifically listed as NYC rates, but she's touring Baltimore - then make it clear that you have already read her site and ads, but were not able to locate the pertinent information. That way, she knows you're serious...
If someone asks about rates, I personally don't assume that he is LE. I assume he's either not serious, or wants to haggle. Even if a guy finds me, say, through a casual mention on a discussion forum, he should be able to find my rates with a very minimal amount of googling. So, if a guy asks me 'How much?' I think either 1) He's too lazy to find/read my site, and therefore not serious, or 2) He has read it, but wants to initiate a conversation like, 'Oh, it's $800? I thought $150.'
Either way, as soon as I hear 'How much' I basically assume that a meeting won't happen.
If she's not posting rates anywhere, that's really unusual and (in my opinion) unprofessional.