This is slightly off of the thread topic, but oh well.

Because we're dealing with time travel, ultimate technology, and a really messed up universe, currency isn't an issue.

Mechs don't need to eat, and they don't need to pay for repairs. They're machines...they can repair and upgrade themselves for the most part. All they need are parts.

Parts are readily available from dead mechs throughout the levels.

While there won't be any currency, there will still be raw materials that can be bartered on the black market. And of course, mechs can carry extra parts to trade for things they really need later on.

The world powers use non-sentient (not alive) mechs to mass produce new parts from raw materials. Consequently, the best way to 'upgrade' your mech is to rampage one of their bases and collect the pieces that fall off of your victims.

Yes, this does mean you'll leave parts behind when you die.

-Nag