It does seem steep, but another thing to consider is that as it is laid out now, you could shield bash with an adamantine shield would bypass hardness on an attack.
Something I have with my primary character in Living Greyhawk is a shield bashing fellow. He has a bashing adamantine shield with, and a +1 ghost touch cold iron shield spike. This way he wields a single item to attack with and two materials to bypass material-dependent DR with and two types of damage (piercing or bludgeoning), no need to draw a new weapon.
It's certainly not something for the warrior who expects to dish out massive amounts of damage, but it is a nifty trick. Especially with the Shield Charge and Shield Slam feats from Complete Warrior.
Regards,
Eric Anondson