A 030 for an A1200 for $80-90AU is quite fair... sure some *cough*blizzard*couth* boards will go for more.
One reason for the conciderably less expencive A1200 accellerator boards compared to A600 is of cause that they are much more common and come in many more flavours and specs. An A600 board, that works, is like finding a needle in the EB-hAYstack.
A second reason might be that many A1200 owners fancey the A4000 and ultimatly end up in that ballpark (upping the prices of these accellerator boards instead). Where's the A600 users tend to do everything in their power to stick by their machine, ignoring things like massive MIPS and RTG, as it holds other values for them. This meens that accellerator boards for A1200 are recycled much more frequently.
Final reason might be that an "Ultimat A600 upgrade" is concidered cooler than an "Ultimat A1200 upgrade" and therefor inspirer more people to try do it than there is actually hardware available to do it with.