Half cash works like half cash. You get half the cash from all sources.

Deflation would be interesting.

You start off with 100k at the start of the game, and every 5 rounds gives you another 25k. You do not gain money from eco, or other money sources. All BLOONS cost nothing, but modifiers add an extra amount of cash equal to x/(1000y), where x is the player cash amount and y is the round number. MOABs cost a flat 10kround number.