As you're progressing through the game, I recommend only buying what is needed for you to complete missions to get through the pro hunts as quickly as possible. Once you do, then you get a 10x credit bonus towards all kills.
If you do this you should only have a few ammo types left to purchase. Then with the 10x bonus hunt the bigger of the big game (bears, moose, etc) as they seem to provide the most creds.
Of course you should do all the missions first (if you're going for the complete) before grinding out on free roams.