The value of Maggette wouldn't be losing Bruce. I don't see why Bruce gets replaced.
We're getting stars in our eyes for Maggette because he's actually athletic and it's been a while since we've seen an athlete in a Spurs' uniform. He'd also stand to fix two of our major problems - rebounding and free throw shooting. Also a third problem we usually have -- stagnant offenses. He'd give us another slasher.
The ostensible cost of Barry+Udrih is not bad, though I doubt that's all it would take, and we'd see at least one very important draft pick go out the window. There are numerous secondary costs (ego, no outside shooting, no defense, injury prone) on top of an expensive, three-year contract.
Hopefully there is another deal on the table other than this one, though I'm sure we'll get some Bostjan Nachbar player buried on the bench instead.