The "confidential" end user list price of a GTX330 is about £2000+VAT. Installation is on top and will range from £300 to replace some existing transponder, to a lot more for a complete brand new setup.
A cheap transponder won't help with the installation cost.
The only way to make this whole business "cheap" is to have a portable transponder, which can be strapped somewhere or installed by the pilot.
The moment an avionics shop is involved, nothing much happens under a grand if it involves drilling holes, fitting aerials, and running wires.
I still can't believe they are going to mandate Mode S for everything that flies. Quite bizzare.