You stated "most" providers don't mention half hour rates.....which seemingly implies you DID see half hour rates from your research.  

Fact is, yes, some providers do offer half hour rates.  Butt in my opinion, the "cost of doing business" in the Vegas environment for a half hour encounter makes it rather impractical.  Not to mention, the ladies in Vegas are just too hott to limit your time with them to a measly 30 mins!  

But to each their own.

You are more likely to get a half hour session as a repeat client than from a provider you have not seen that does not publicly list half hour rates.

Try researching Pornstar/PSE Providers as there is more of a chance of listed half hour rates.