You don't really get alot of money when you play the game through just the main stages. If you find a suit or some items that you really want to have, you should go to the training levels. You won't get alot in the beginning training levels, but when you get to level 4 in the training levels, the money will start rolling in. The thing is, you have to play it over and over to keep increasing how much you earn, and perform well, but if you do, you won't run short on money.

To unlock the Blackbird Suit, just beat the game on Normal or Easy.

To unlock extra levels for Trial Missions, finish each Trial Level with a rating of "Professional". To do this, complete at least 13 of 15 rooms, destroy all enemies, complete the level with a fast time, and get as many "No Damage Clears" as possible (usually 6+). You can unlock up to 5 levels total per Trial-completing Level 5 awards 20,000 points.

Beat the game twice with the same same save file. Doing so will unlock the Papillon Suit.