A normal corydoras is reasonable at 1-3 dollars. A pygmy, or other semi-Rare cory is reasonable from 3-15 dollars, though 15 is really high. A rare corydoras can be 20+, I've even seen some newly developed color morphs go for 40!
The ones at my lfs are about 8 dollars. On aquabid I can get 10 for $25 plus $15 priority shipping. So it would be four dollars per cory. Not sure if the shipping stress is worth it. Can they handle it?
I heard guppys were not safe becuse their tails are like bettas (mistaken identity) I have a lot of ghost shrimp and 2 ADF in my 20 gallon sorority/communty tank I did have danios but had to move them because they are pigs lol