Check the garbage sugars and transfats on any of the 'prepared' tuna products. Some are just fine, but keep an eye out for it.
Broke? Get your tuna in #10 cans, split it up into portions and freeze it. NO muss, no fuss, and MUCH MUCH cheaper.
Cheapest by FAR protein you can buy is big bulk whey, but not many of us can deal with 50kg of pure protein. Next in line portion per portion is bulk frozen eggwhites. Bulk frozenchicken breasts are REAL cheap too...
GET THEM FROM A RESTAURANT SUPPLY HOUSE! Just tell them you are a home caterer and bring CASH with you, be sure to tell them you pay in CASH.
In order to make the most of saving day to day you will need a freezer that can handle the burden.