I can't believe I have not imparted my wisdom yet in this thread.
If you are in Budapest and work with amateur models 5k is enough to LAUNCH a paysite. If you want to turn older content into a paysite then 5k is more than enough. BUT (and it's a big ol' butt) if you want to shoot in 4k (or HD) and have the hottest models, sets, "look", etc then yeah add a zero (or more) to that 5k.
So it really depends on how commited you are. Where there's a will there's a way but more $ would be needed if you are starting completely from scratch. Because let's not forget the cost/skills needed for hosting, payments, design, webmastering, traffic generation, etc.
Best of luck!