not knowing the value of the item you are discussing, I would say to go with the 70 % rule... if it's 70 % of the original price, or less, it's worth taking a shot. the companies who rebuild these pay alot more for labor (US repair labor versus overseas assembly labor) than they do for pieceparts, so a refurb unit will generally (not always but generally) be opened up, the troublesome piecepart completely replaced, and then bolted back together for re-sale. they simply can;t afford to troubleshoot down at the piecepart level.
the odds are your unit would be as good as new. keep in mind that good as new is subjective ( a bad production run of a specific piecepart can go a long way, even into the rebuild part of unit life), so check a few of the posting boards to see if there is an ongoing theme of customer complaints about your unit. if there's not alot of whining going on, the problem with a rebuilt unit is very likely a freak and the surgery would probably be a success. if there are a number of specific complaints, you might want to reconsider, as there's no guarantee either a rebuild or new unit would be worth much.
if there is a secondary cause to the primary problem or if they had a bad batch of the given piecepart, it will be very likely to occur again within the 90 day window.