What only one or two people recognize in this thread are TH level and time.
As a TH8 or below, it is very easy to dump gold into walls even hours before a builder is free and raid back the gold in order to pay for the next upgrade. Even if you only play 1-3 hours a day.
As a TH9 or above, this becomes a different story. Upgrades in 3m+ and based on averaging (per the guy above) .5m per hour, you're talking about significant raiding time based on what you have left. That means you maybe only dump gold worth 1-2 walls rather than 4-6.
I too found it very difficult to upgrade walls at TH9 (current TH) while upgrading defenses like a mad-man. I kept 5 builders busy with very little down time Oct - Jan and now that I have completed all upgrades, I am sitting with 5 builders free, upgrading walls every 1m raided and heroes every time they wake up.
The big caveat here is that if you can raid/boost for hours everyday (like the people that claim they can dump gold into walls), you can likely keep builders busy, upgrade walls as well as heroes throughout the life of your TH9/TH10. It's pretty unrealistic for those with jobs or the semi-casual gamer..



