Not really a knock off, just an amalgam of old and new: The base is antique, the drawers and top are modern, the head is modern. The head says "Singer Sewing Machine Company" which is a dead giveaway that it's new.
The legs say "The Singer Manfg. Co." and have the long bobbin trademark, so they're original.
The machine was probably sold as a brand new repro machine and someone has put it onto antique legs.
I've seen original cast iron bases go for almost $100 by themselves (for people to make into a table), so it's not a bad price if you don't mind Chinese reproductions.