Finasteride is the main active ingredient in Propecia as well as Proscar and various other drugs - it's probably expensive because
1) It's marketed as a cosmetics drug. Your health insurance is probably not going to cover it;
2) Merck had the original patent on Finasteride so they can charge exorbitant prices for the exact same drug marketed as something else in a different dosage.
$12 a month? I personally use Propecia and it's costing me a leg and an arm (something like 60 dollars each month). As some people pointed out it might cost more to make fakes than to produce real finasteride, so. In any case, always be prudent.
The hair you have prior to using finasteride should not be affected by it; what the drug should do is that it stops the majority (but not all) of the conversion of testosterone into DHT. So hair that is not being affected by DHT will still be there with or without the finasteride. Now what you should expect to happen should you have any gains and decide to quit finasteride for whatever reason is three things:
1) You have hair that is unharmed by DHT, they will not fall out;
2) Hair that is being miniaturized by DHT (thick or not) will revert back or be a little bit thinner;
3) Hair that you gained back will fall out.
Now, I'm no expert on this subject; on these forums you will find a spectrum of gains from finasteride. Just realize that the younger you are the more likely the benefits you derive will be good. That being said, there have been a few older gents for whom treatment has done quite well on.
Everyone responds differently. What most posters on this forum will say, however, is take pictures, stick to a treatment, wait, take more pictures while you wait for a year, and assess your situation at the end of the year, to see if something is working for you, or you should modify your regimen, etc.
Good luck.