Depends on which one it is. There are two Red Steel box sets, if you weren't aware:
2504 Red Steel Audio CD Campaign Expansion, Boxed Set
9500 Red Steel, Savage Baronies Audio CD Accessory
I would suspect that a lot of time people think that the 9500 boxed set, which usually goes for $10-15 in
SW, is the only one. The 2504 boxed set is usually harder to find, especially in shrink. I would suggest that it is worth a little more than 9500, perhaps $20-25.