I have has a Garmin GDL-39 for ADS-B in for a couple of years now (about $500..00.) I recently added ADS-B out by purchasing the Dynon SV 261 mode S transponder and 2020 compliant GPS for my existing Advanced 5600 EFIS for $2600.00. All installed by me. So the complete ADS-B system was about $3300.00. Plus $130.00 to electronic shop for certification. Garmin unit is also about t$3300.00 with compliant internal GPS, but it must be installed by a certified shop for more dollars. Here in the puget sound area there is not much airspace where a legal flight will be possible after 2020 so for most operations a ADS-B out system will be a necessity.