Ours was fitted a couple of weeks ago - the Garmin unit.
The cost of the unit was £2,000 plus VAT. The cost of fitting will vary a great deal. In our case the fitting was complicated by a number of issues. I would have thought anything in the range £400 to £800 would be reasonable.
Given good access, there is nothing complicated about the fit, but a new tray will probably be required.
The unit does not come with the encoder, but you will almost certainly be able to use your existing encoder.
Advantages so far - the unit displays the FL being received from the encoder, a useful means of verifying the encoder is functioning correctly, and has a "nice" digital flight timer. There are also audible altitude warnings if you wish.
The disadvantages - both Gatwich and Fanborough "admitted" they are unable to receive mode S information so no one knows we have had it fitted!
Dont forget you will also have to obtain an aircraft specific code from the CAA and the installation will almost certainly result in a minor mod. fee - which incidentally you would have thought the CAA would have waived given that it was a legislative requirement that the unit be fitted in the first instance - and their was me thinking the CAA should have .. .. ..