I had a GDL88, GTN 750, 406MHZ ELT, and a True Blue TSO duel USB charger installed in my M20 F this summer. Here is what it all cost. This is from the actual bill from the avionics shop. (The exception was my transponder needing service and a pitot static check were extra.) I could not be more pleased with how it turned out.
Description of work Equipment CostRemove KY197
Remove KNS80
Removed GPS60
Removed WX-8
Installation of the following:
GTN750 $13,943
GDL88 3,296
Dual USB 375
ACK/E-04 674
Misc - wire, crimps, splices, etc. 400
Sub Total $18,688
Sales Tax 1261
Equipment Total with Tax 19,949
Total Labor 85 hrs @ $60/hr 5,100
Grand Total $25,049