All of the Deathnote cosplays (except Mello) are relatively simple and easy to find.
Honestly, i think you could do an L cosplay for under $30. Just go to any WalMart and get some light, baggy jeans and a loose white shirt.
For Kiira, the pants, shoes and white dress shirt should be pretty easy. The cost might be a bit harder, but agter a few coat shops, you'll probably find one. The tie varies depending on what shade of red you want.