9mms are the cheapest centerfire pistol to shoot. I buy winchester value pack of 100 rds for $10 at Wallyworld.
There are a lot of good 9mms out there- Berettas, Highpowers, Glocks, XD-9s.

.45 pistols are more expensive to buy and more expensive to shoot. They can be made real cheap to shoot if you reload. It costs me about $.05 per round or less to shoot mine- $.02 for a primer, $.015 for 5.5 gr of w231, bullets cast from wheelweights that I get for free, brass that you will lose long before you wear it out.

If you just want something cheap and powerful and have a little bit of a WTF (is that)? factor try one of these:

http://aimsurplus.com/acatalog/CZ52_7_62x25_Pistol.html

and buy a 1200 rd case of ammo for $120


Makarovs are also good cheap guns that shoot cheap ammo.