You can get a prescription for Proscar, which is 5mg finasteride instead of 1mg like Propecia. Split it into 4 pills, or even take a half of the 1/4th every day. There are some studies showing the effects of smaller doses being almost as good as 1mg with less side effects. 30 pills of Proscar, which equate to 4 months supply is like $75 from Wal-Mart, so it's about $17 a month.
I suppose you have to take it as long as you want to retain your hair, and then get off of it when you are fed up or when something better comes along.
I dunno. I got a prescription yesterday from my doctor, but I haven't decided if I'm going to fill it yet.