An RM60 intact with good pulleys and freshly running is worth the $700 quoted, IMHO. New is $2700 and yes I do sell them at that (2 new from our shop in the past 12 months).
An older M48 etc. is perhaps worth a fair bit less.
If this is missing all the mule drive componentry, that is really expensive to replace and likely will not be located used. If you drop me an Email with the details (photo?) on what's missing, I can give you some guidance. If only the main support frame, then it may be worth it.
The RM60 is my personal favorite ... used on a 1 1/2 acre property with plenty of trees. Amazing reduction in cutting time vs. RM48. But I do keep both in service, using the RM48 mostly for mulching and vacuum duty.