Chris has stated that the value of a recently-restored HEP would generally be about 15% below the owner's total cost in the game--which is basically therefore, the unboxing privilege. (Otherwise, someone could just buy a base game, send it to HEP, and unbox one themselves for the same cost.)
That said, that's really just a guideline--as I recall a low-play HEP CFTBL (just months out of restoration) that had to be marked down three times last year--from $7500, to $6900, till someone finally bought it near the last price reduction of $6400 (which is obviously a lot less than 15% of the owner's total cost).
Really depends on timing as much as anything else: the number of high end potential buyers actively looking when you happen to be selling your machine.