The formula that is used to calculate how much it costs to build 1 acre was changed from

3*land+1500

which is a linear formula, to

3*(max(1,land-1500)^1.05)+1500

on 2013-04-06 ( http://www.earthempires.com/...hanges-23941?t=1376061607 ). This is a polynomial formula (not exponential, but they both mean the same thing to most people). This was subsequently reduced to

3*(max(1,land-750)^1.03)+1500

later on 2013-08-06 ( http://www.earthempires.com/...resets-25801?t=1382991109 ), which is cheaper, but still significantly more expensive than the original costs. This meant that converting to MBRs incur a much higher rebuilding cost (and higher tear-down cost), to the effect that it is too costly to perform compared to any benefits gained as a TMBR.

This set of changes were aimed to combat landtrading on the Alliance server, in response to this reset ( http://www.earthempires.com/alliance/20/ranks ) where the top 33 players were ALL landtraders when LaF decided to show the world how Landtrading should be done after years of arguing that "Landtrading is the bane of this game because it is overpowered and that it shouldn't be allowed". Failing to convince anyone, LaF spent 1 reset proving just how broken it is when every other clan out there suddenly has 2x-3x as much landtrading partners to trade with.