This is where say you have a level 2 rocket turret and you and your friend had 300 money each. It would be nice for me to go in and put 300 in to the turret lowering the cost TILL it gets upgraded to 300. It promotes communication and teamwork ill let you guys decide if it has balance issues.
I think the main problem with this is that most people aren't going to put money in because they won't gain anything out of it, just like the way the person who built the first 2 levels of a turret doesn't get anything if someone else upgrades it to 3. It's a good idea in theory, but it would almost never be used.
