I'm not trying to be a smart ass, but it's worth whatever somebody is willing to pay for it. I've been following the vintage Sunn market for a few years, and these things are all over the place price wise. Some times a nice clean piece goes for next to nothing, and then a couple of days later butchered up piece of crap will bring top dollar.
If you want to sell it, you should set an asking price based on what it's worth to you, and see what you get offered. If nobody makes offers, you were too high on your asking price. Then you could lower your asking price, or wait a week or two and put it up again. Big (4 power tube) old tube Sunn amps like yours go through eBay occasionally, if you check completed auctions you might get a price range for what people were willing to pay, that is IF they actually paid for the amp after they won!