I would say it is a great price. Prices have been dropping lately, but the best I've seen is in th $650 to $750 range. I bought one about a year and change ago and I paid $925 at that time, ouch!
Have you run the serial number by Armalite? Their web site will confirm the preban status, but you have to contact them directly to have them confirm that lower left the factory as a complete rifle prior to the ban.
If not that, then you should get the seller to document it was built up prior to the ban. Of course none of this is mandatory, it just gives me peace of mind that all of my prebans are confirmed by the Manufacturer or seller. But thats just me!
Enjoy your well priced lower....
Cheers.