Longman Optical came back with an estimate and wow you're right Mick, the cost of coating is $180 AUS ($142 USD - 79 roughly) that's for all 8 surfaces. That assumes the elements are sent disassembled, otherwise there's an additional cost of $160 AUS.

This is for single layer MgF2 coating which they say is sufficient. This becomes a viable option at those prices.