Yatzmich:
the ad says clearly, for the first 12 months once your in your in. if a girl chooses to stay in after 12 months (presumably because she appreciaites the extra attention she is recieving and thinks it gives her a better shot to get married, then she brings herself current).
m in Israel:
the $500 is just as guranteed as the shadchanus money. if a girls wants out (or gets engaged via a shadchan who didn’t get her name from the list) she gets the money back. in addition the first 50 girls that sign up are not being asked to put the $500 in.