I'm not actually sure how much the H290 flares are in Australia, but they will be more than $50. They retail something like $40 each in the US, so add on exchange rate + overseas import costs etc etc, its more than $50 each AUS. The 2370s are big though, about ~17.75" wide and almost 7" high, the Eminence H290 is only 11.4" wide by 6.4" high. Just means your box won't be so compact, doesn't matter though. My midrange horn is about this wide so the box has to be like that anyway.

You could active filter the system. Mines active filtered between the woofer/midrange horn, and then the Pi passive crossover between midrange horn/HF horn. I'd reccomend you do it this way, so the tweeter compensation circuit is already worked out for you.

Eminence 12" would definitly be an inexpensive way to go. Last time I checked, the Delta 15 was $225 AU each, so the Delta 12 would be even less, not bad. I remember you talking about the 18 Sound drivers, would definitly be a great way to go, but they also cost more than the Eminence.