Partial Team Upgrades
I think the game would end up moving forward upgrade wise a lot faster if you were able to do partial upgrades.
IE: Right click the upgrade and dedicate however much gold you wish to contribute to that upgrade. The upgrade could fill up a color to show the % that is paid and another team-mate could just left click to finish it.
Would be great to use that last 800 you had after a run back or whatnot and let the team work together a bit more if they REALLY need an upgrade no one by themselves could afford.