That's going to cost GA a lot of money. More than Mode S in most cases.
A basic but good quality 8.33 radio with NAV like the KX165A is $4000 or so (I put one in fairly recently).
There are some cheaper options but they will require significant rewiring - as will fitting a 165A in place of a 155 (non A) for example.
I guess most people will throw in a GNS430 or 430W, bought off Ebay - if they haven't got one already. But that's more rewiring.
For the lower airways, this is a crazy proposal.
Won't one need two 8.33 radios for IFR?