around $40-$45 or so for a $100 Sahara plaque. I've occasionally seen them go in the high $30s, and I think there is one currently in an eBay store with a 'buy it now' price of $50.
Lower serial numbers (say, below 500 or 200, so that in theory you could put together a matched set with other denominations) tend to go for a premium, although I couldn't say how much.
Of course, I am far from an expert in such matters, so all of the foregoing remains my humble opinion.
Brent J. Jensen
orbis non sufficit