The difference between Biggies estimate and Vermonsters estimate is the difference between a Georgia appraisal and a Vermont appraisal. Apparently they are worth a lot more in Vermont.
Actually, the rule of thumb is that they are worth what someone will pay for them...no more, no less.
The only way to really know is to put it up on e8ay with better pictures than you posted here and a really accurate description, and let the bidders set the price.
A decent guide is to go to e8ay and searck in the archery section for Bear SSuper Magnum and click on compleated items. It will show you what they have sold for over the last 50-60 days. Now yours may bring a bit more or less than those sold depending on the year, condition, and how many others are available for sale at the time yours is up.
With the economy in the toilet right now, it is a time for buying, not selling. There are some great deals out there right now going relatively cheap, and that is your competition...even bows other than Super Mags.
Best practical answer I can give.
However, if you play hardball with Biggie, I'll bet you can push im to $30, if shipping is included.