proper vipers do cost a few quid.
there are several different size and fitting configs about. Make sure you get the right ones etc.
The set of vipes and pipes I had on my 851 were proper corse items, of an early race bike, as such were a good upgrade on the bike, but some of later systems use larger than 50mm diameter pipe. also the silencer pipe length can vary quite a bit.
I paid £600.00 (and that was used) just for the nest of vipers last year, let alone the silencers etc.
you may find it cheaper to buy a proper set in the long run.