Pricing information for US:
A/C assembly (and actuator [optional]) from the CX-30 can be purchased at any US parts department for $50 after taxes. The rest (ducts + rear panel) can be purchased through a German parts reseller, with a total of 450 euros after shipping and VAT (~$530 USD).
Total cost appears to be ~$600.
Haven't pulled the trigger myself (yet), as the cost was much higher than I anticipated.
Compatibility update:
I took my console apart to look at the rear vent hole that's plugged. I tried to pry out the plugged portion with a flathead, but had no luck. It looks like it's not a removable stopper, but rather plastic that's part of the whole assembly. We're probably going to have to cut a hole.
A/C unit assembly part numbers corroborate that there is a difference (BDT
W61130A for Mazda 3 vs. BDT
S61130A for CX-30).