Inflation is inflation. But it also depends on which market you're looking at because inflation in certain markets may be higher or lower than the general.
Using this:
http://www.bankofcanada.ca/rates/related/inflation-calculator/
$50 in 1972 would equal $274 today.
Keep in mind however this does not account for that specific field. I would expect the price for upper class escorts to have gone up at least ten fold due to social conditions not limited to HIV and lower crime rates.
As someone has mentioned, I think depending on your social group, casual sex in the 1970s was indeed higher than today. Despite the fact the opposite is often argued by the mainstream, teen pregnancy rates do not correlate with higher number of sexual partners. Not to mention that from everything I've heard from people who lived in that time period, if you were in a particular scene, you got a lot more exposure than the average joe.
I do agree the rate was probably $50 an hour, and that would be a fair rate for 1972.
But then I heard this old line how back in the day, whenever that was, you could denote service within the cash margin of $100, with $10 being the cost of a LD, $20 for a HJ, $50 for a Blow and $100 for FS.