i am vexed at the moment by the filtration costs.
What is it that is clogging your filters? Dust? Pollen? Small animals??
We're in a similar situation to you, though even
better as we're blessed by even cooler ambients
and never see 100°F, or even close to it. So
we're using make-up air 12 months of the year and
really only go fully mechanical in cooling on
summer afternoons.
We change our filters monthly most of the year,
and in spring when the pollen count skyrockets we
change from box-pleat to bag filters and change
them every week or so.
Dust is never an issue up here... it rains way too much.
--chuck @ d.f in seattle