There was also the potential cost of TAKING the opportunity in that by trading down to 15, they miss out on BOTH Whitner AND Bunckley, leaving them with the 3rd best S (Allen, who is definitely not as good as Whitner), and crap for value. McCargo would have been too high at that time and no one else was worth a pick at 15.
Marv and co. handled this perfectly. Maybe in an alternate universe they were able to trade down a spot or two and still get who they wanted, but when Oakland took Huff, the Bills HAD to do what they did.