The quotes that i am getting for a panel mounted Mode S are between £2800 (Garmin) and £3100 (King)+ £170 for the Alt encoder + VAT, I suspect that you could dirve the price down by about £200 with one other type of transponder but that is all that is on the market for panel mounted units.
This is a far cry from the CAA estimate of £1700 - £1800 for a panel mounted LAST unit but this is all that is on the market at the moment.
For you interest take a look at
www.casa.gov.au/pilots/download/ADS-B.pdf
This is what we should be getting if we have to go down the mode S road.