They're constant yes. The only thing MBs change on turn ratio is that they do actually lower military upkeep expenses a small amount and that has diminishing returns.

For optimum private as MBR you'll want to build your MBs, then demolish your CS, then drop all acres continuously so you're build is 100% MB. That's the best prices whether you have 20 MBs or 20,000. The market will refill based on the number of acres you have, so a good amount is needed for an entire destock as MBR unless you intend to burn oil to produce units. The more MBs you have in quantity and not as a ratio to your acres only serves to lower expenses.

Imo theo mbr could in theory still be a viable strat as you can nearly double income on purchases. I haven't seen someone run a true fatty mbr in forever though. Just banks (the 100 acre stay in protection TMBR that vicvixvi sometimes runs). I'd be curious to see an actual mbr in action.