upgrades in between maps will be bought with gold. Prosperity is basically a high-score and it'll carry through maps for a full run. So ultimately, generating prosperity will be an optimization/minmaxxing issue for people wanting high scores.
Will post some updates soon, as I've currently gotten good UI/UX feedback and working on revamping and improving the early experience a bit (and adding a bit of a playable tutorial).
Thanks a lot for playing!