It's probably a good price. From posts I've seen here on TBN you can expect a new one to be more than $5,000, maybe $6,000 prices seem to be going up.
Be careful you don't break your NH, when that Kubota backhoe goes on a Kubota tractor, the top 3-point tie is reinforced and the frame back of the axle is changed to reinforce the lower attachment points. I'd check with your NH dealer and see if there is a similar reinforcing kit for your TC24 when a backhoe is put on. NH may not need the lower frame reinforcers but probably will need the top attachment point reinforced.
The top 3ph pins for the backhoe are hardened. Regular pins will deform and be hard to pull out. If you're not sure what you're getting, buy a set a hardened pins and keep them with the backhoe when it's off the tractor.
Make sure you get the manual with the backhoe. There are a lot of adjustments on the BH to mate to different model Kubotas. You'll need the manual as a guide to try and figure out which different sets of holes and brace lengths to use in your situation.
Keep the loader on when operating with the backhoe. Besides needing the weight to offset the backhoe. The bucket set cutting edge down will help stabalize the tractor when digging.
Happy digging!