I think it is way too costly.
We have had this system for six years now and replace the HEPA about every 18 months. Vacuum it off with each pre-filter change and extend the life. Tried another system and did about half as good as this one for almost the same cost!
Check out FiltersUSA.com, FiltersFast.com or NationalAllergy.com for OEM as well as lesser cost aftermarket replacement filters. I've tried both, the aftermarket has a frame made of cardboard instead of plastic so its not as tight a fit, but at half the cost you decide. Not really sure if there is a performance loss in going with the after market replacement. I continue to buy the Whirlpool OEM filters (1183051K) as long as there available and under $80 / filter.
No worries. If you are looking for replacement filters, enter your unit's model number in the PartsDirect website. The stores don't carry the older units' parts any longer so you won't be able to order them here. But PartsDirect has all the replacement parts.
Fits fine in my 10 year old kenmore air cleaner, seems to work well. Wish they were cheaper, though and they never seem to go on sale.