Back in 2000 they still had a metric butt load of 500/1000/2000 pound bombs from the Vietnam era.
That said, the only way missiles are cheaper than arty for putting warhead on forehead is to get a contractor to come up with a new precision/stealth/hypersonic/cyber-ready/environmentally-friendly/insert-latest-buzzword-here ‘wonder-weapon’.
$3100 was the new production price in 2000, IIRC, not the acquisition cost of older ones. A lot of the old Vietnam-era bombs ended up having to be disposed of instead of dropped in combat, too. https://www.epa.gov/sites/production/files/2015-05/documents/9421083.pdf