I work at a Bernina dealership. The most we have sold a used 830 for, was $550, and it had 12 presser feet (including a walking foot), the case, knee lift, accessory box, sew table, bobbins, etc. The 800 and 810, would go for less. I fixed an 807 that I got from the thrift store, and sold it for $125. You might be able to get up to $250 for the 810 if it has lots of accessories. The thing you want to be sure of on these older Berninas is that the camstack and vertical shaft gears are not cracked. Also, make sure the stitch selector lever (on the top, right), and the needle position knob (in the center of the stitch width knob) are not gummed up -- which happens when a machine has been sitting unused for a long time.
Good Luck with selling your machines!