The shop I used to work for was an authorized US Amps dealer. We sold that amp for around $650 IIRC.
The 5500 was the predecessor to the 5600X. The 5500X was an eXterminator model that was in the eXterminator heatsink (flat black powder coat with laser etched logo). Was a very solid amp and we sold quite a few until the 5600X's came out. I used to run a 5600X in my car back in the day. Loved it.