Upgrading Overhaul

Make upgrade costs proportional to unit strength.

A unit at 50% health should cost 50% less to upgrade.

The game could work either way:

1. Upgrades have the side-effect of healing, thus they cost more for damaged units.

2. Upgrades do NOT have the side-effect of healing, thus they cost less for damaged units.

Pick either one, it would be fine!

What players like myself do not like is the fact that all upgrades cost the same but the "value" of the upgrade is less for damaged units. This hurts inexperienced players the most, because they think they need to upgrade everything as soon as they can. Just like they research and build everything possible.