Similar to the cash you earn in Horde mode, in Beast mode, you build up cash to unlock new tiers of Beasts. You also use it to purchase any given Beast for a round (or until you die). Between rounds, if you survive without dying, you recover a percentage of the cash you spent on your Beast. This depends on how much damage it takes during the round.
Dealing damage and destroying fortifications and targets increases your cash. Likewise, performing certain support actions, such as healing your teammates with the Kantus, increases your cash. In general, assuming your team makes a no-death run through all twelve waves, you will just barely have enough cash to buy one or more Berserkers by the final wave. If your team wipes out on a wave, you may have the chance to build up a larger reserve.