MSA detects Nod's stealth units, not spies.
And Mines at the entrance of buildings don't stop spies from placing beacons at the roof. Even if you manage to mine all entrances to your base without blowing the limit, they only need one spy with a repair tool to disarm them.
I've seen only once a protection against spies: A Nod-Team at file facility had completely surrounded their PP with barriers built by pioneers. But it would have also prevented them from repairing it if Tanks would have attacked PP.
You already invalidated those arguments yourself:
There are maps where money is nearly unlimited. The maps you enlisted are exactly those where it happens most of the time. I'm also against reducing the money on those maps since it allows also enourmous battles with very expensive tanks, battleships, having all upgrades,... Taking all those away to allowing some people playing an more-than-OP unit is bullshit.
Players might not be in range to get it in time, might have no repair tool.
It needs at least a specialized countermeasure for spies, how about introducing RA's guard dogs? For example allowing pioneers to build a dog house which spawn a dog killing the spy when in range.
If you still think, spies are not OP only because of their price, just revoke the ezek nerf, make it ignored by AGT and other defences and make it 5000 credits.