Agree with copul8ter - The P411 screening keeps a lot of us away - I understand 100% why you girls want/need to do it.
I looked into it - and I am an evening guy - only because my wife wants to be apart - in no way would I ever do this without her. She is also very inexperienced with women - loves the idea, likes to talk about it - but very nervous about it. So I can't really plan for it - I have to catch her in the right mode and at a time she is willing which you don't know how many times has failed me.
So availability and ease of access is usually my issue.
Price can always play a part - agreed you get what you pay for - but sometimes you can get lucky