Global repair

The problem I'm facing is that in my map a lot of buildings can take damage at once and it gets old running from building to building repairing them and doing not much else.

So I need something to mass-repair all your buildings on the map. It should take money depending on the amount of HP recovered.

I've thought up two solutions and can't get either to work.

Making a spell cost money (and fail if there is not enough €€€ in your stash) is one thing, but what if I only want to spend €25 on repairs and spend the other €45 on a tower upgrade instead of blowing it all on repairs to bring everything up to showroom condition and then leak because I didn't have the €45 upgrade? In other words, I need a way to perform a "minor repair".

I could use repair-o-drones, but that would be a micro nightmare unless I could tell them by trigger to repair different towers, and stop when I hit a button.

The ideal solution would be a building with a "repair-everything-over-time-and-take-money-per-point-repaired" area effect aura that can be turned on and off. Of course, that's way beyond my reach.

Help!