I'm a glutton for punishment so I'm going to post some numbers here for you guys to read and ponder. Here is the cost and time to upgrade th 7 and 8 straight from the wiki:
| 7 |
67,441,000 |
30,109,450 |
105,000 |
276d 8h 53m |
79,575 |
$579.91 |
| 8 |
223,669,050 |
71,298,550 |
382,500 |
562d |
185,708 |
$1334.84 |
For th7, that 276 days equates to 55 days with 5 builder at th7 and you need to get 67 million gold to max it out. At that rate you need to accumulate 1,226,200 a day to max everything in the fastest time possible without gemming. I will say that again: you only need to farm 1,226,200 a day to max out th7. For th8 the 562 days equals 112 days with 5 builders so you need to farm 1,979,372 per day to max .
Why would you waste 20 days upgrading mines/collectors when you can farm that? You can't store gold in this game. There's only so much you can spend/hold.
At th9 it becomes 6 million per day. So unless you are a hardcore farmer, this is actually the time when you should max out your mines.
Have a pleasant day. :-D