OBD2 P72 ECU's are practically being given away. You can find them for around $50 on ebay.

OBD1 P72 ECU's are quite expensive and pretty hard to find in non-chipped condition. You can find a few on ebay, but they will run around $220 or so. I just got an OBD1 P72 from some guy off Honda-tech for $165, so you can find them cheaper, but it is pretty hard to do so.