If you're going to suggest not upgrading them, the obvious extension is to never build them at all.
I have had 8 months of L11 mines, or very close to it. (now TH9)
If they were L10 mines instead I would have had 500 less gold per hour, that's 500 (gold) x6 (mines) x24 (hours) x182(days) = 17.5M gold.
You don't want 15M gold? (I am assuming you lose some to raids)
That's entirely your business of course, but it makes good sense to me.
The payoff point for each level is listed on the excellent wiki, here.
http://clashofclans.wikia.com/wiki/Gold_Mine