Look up the prices of the bits as new, a very rough guide would be 50% to 75% of new prices, but very subjective.....
Don't under estimate the value of the chuck, the additional jaws, the individual tools, and even the wood blanks, there can be a lot of value in those, easily as much or more than the lathe itself.
Bottom line is whether it's worth it to you of course !
Cheers, Paul