I am not aware that anyone makes an after market DC15 HEPA filter; as they were made in relatively small numbers (compared to other Dyson models) it likely wouldn't be commercially viable to do so.
Accordingly, Dyson seize that opportunity with both hands, which is why they are about thirty quid.
I find that when Dyson control the after-market market with spares for a low selling model machine, they tend to shamelessly empty the pockets of those who want to buy the spares. It matters not one wit where you buy genuine stuff in those circumstances, prices will be pretty similar at any site (give or take two or three quid) because Dyson do not bulk discount or do deals. One reason we can price match anyone on genuine Dyson stuff is because we all get pretty much the same discount from listed retail. Add P&P, card fees and a margin for profit, and it is challenging
(but entirely possible) to beat Dyson's retail price.
This is why sites like espares
charge more than Dyson themselves do - so they actually make a profit.
This is also why we constantly press for lower prices - because we expect to be able to beat sites like espares
and other large parts sites because we are brand specific
and thus likely buy more than they do; so if anyone is getting discounts, we hope to be somewhere near the top of the tree. If we can pass discounts along - we do.