Leaving information here incase editing the game's page delete's it from the GMTK submission list... It shouldn't do, but I am taking no chances. The description is very limited because I uploaded the game the last few minutes available to me. (I updated the description in the 1-hour extension).
Prices are as follows:
Plains - 0 Gold
Walls - 2 Gold
Turret - 5 Gold
Bog - 3 Gold
Level Upgrade: 3 gold per current level (e.g. Upgrading from 5 to 6 costs 15 gold)