If you pretty much don't PVP and don't research PVP related modules, the only other thing to spend credits on is energy module upkeep. I'm still upgrading to +8 structures. For +10 structures, with an income of 25m/hr and an upkeep of 50m/day I could only hope to keep up with how fast I settle planets, rebuilding the old ones would be a very slow process until the mining/hr and mineral quality caught up. I pretty much have to chill out on PVP and NPCs to even save the repair costs to build the +8's within a week. The difference is pretty big though and +10's are the same difference. I might actually let my ship modules remain somewhat crappy untill I get +10 structures and wormhole relays.

your average is wrong, at lvl 200 you get 2M creds for 100 minerals ... So your 400 minerals are worth 8M creds, and probably more at your lvl.

I think we're looking at a problem where none exists.

Minerals are just like energy - you want more so you can do more. When it's spent, it's spent. It's not about gaining profit.

The idea of upgrading planetary buildings is:
-to have more production capability (freeing space for production buildings)
-to make production source more stable (freeing space for more defense)

The only real problems are:

1) Being able to support your ship's maintenance and have enough minerals for upgrading while also having enough reserves for certain events. If you are having trouble just for maintaining upkeep then you are building more than you can support.
2) Finding ways to lower your mineral production in favor of another production type because you have way more than enough you need. Imagine what would happen if you can regain your expenses in a matter of days or even weeks.