What are good inexpensive tropical fish for a 40 gallon  aquarium?

Guppies - need at least 3 of their own kind with all males or 2 females per male. Have long, flowing fins so shouldn't be kept with fin nippers

Platies - same group and sex ratio requirements as Guppies, need a pH of 7.0 or higher

Harlequin Rasboras - need groups of at least 6 of their own kind, excellent community fish

Corydoras Catfish - need at least 4 of their own kind and a smooth sand substrate

Zebra Danios - will fin nip, so shouldn't be kept with long-finned fish. Need groups of 8 or more

most types of Tetras - need at least 6 of their own specific species, some species fin nip

Tetras are a good option if you want small fish, and also a couple of catfish.

I would keep away from swordtails, since they are known jumpers-so are tri color sharks. 

Couple Cory cats to keep it clean, maybe some schooling fish like Danio's, Maybe some barbs-but NOT tinfoil barbs-they get too big. I've had few jellybean parrots too, but they Can get big. 

Guppies, platys and sword tails.  Add some catfish to keep the tank clean and some plants to balance the water. 

Get you momma's colander and use that to catch guppies at the sewage canal. There, free.