Since i had hit 25K there was no fees for me and i entered usually 2-3 lounges per day just to check them out and compare them to each other. IF I was below 25K I would imagine I would be charged 1K per entry every time.

If you are going to just pick 1 lounge to visit while you're in Vegas I would go with CP or PH as both are the larger lounges.
PH has restrooms inside and lots of TV to watch sports.
I would avoid Flamingo lounge.