I just did a quick search for Serial to Bluetooth adapters and found two, both in the $100 price range so the scantool price doesn't seem so unreasonable (plus scantool will guarantee compatability with the Elmscan 5).

I guess you have to realise that Serial to Bluetooth conversion is probably a nice requirement (unlike USB/Bluetooth) hence the higher price as fewer units will be shifted. As you suggest, scantool are only doing what they can with what is available.