I've pinpointed the exact bug where block purchase refunds are vanishing.
Procedures to replicate on v9.1.0, Bukkit 1.2.5R1:

1.) Place First Field (money deducted)
2.) Enter Cuboid Draw Mode
3.) Place X more field blocks, tested with same type so far(money deducted)
4.) Destroy the FIRST FIELD during cuboid Draw mode
5.) Lose Cost*X blocks

Basically only the original placed field's money cost is refunded. Any extra fieldblock costs added on during draw mode mixing/volume increasing will be lost if you destroy the first fieldblock. If you destroy subsequently added blocks BEFORE destroying the original fieldblock, you can still get your money back.

There should probably be a routine somewhere to sum up the number of blocks inside of a cuboid drawing mode and return that cost. However, there is currently two different behaviors when destroying blocks in drawmode.
1.) Destroy the first field (the one you clicked to get draw mode) and ALL subsequently added fields will break and turn back into blocks
2.) Destroy any fieldblock other than the first one and only that field block will be turned back into a block

the bug is come back in the latest build for 1.3.1

This was fixed