Some basic damage ratings for GP (standard) bombs:
4,000 lb - 3,125 damage (there is no actual 4k GP bomb in game anymore)
2,000 lb - 1,718 damage
1,000 lb - 1,000 damage
500 lb - 550 damage
250 lb - 313 damage
100 lb - 156 damage
AP bombs: 150% damage, 20% radius
SAP bombs: 125% damage, 33% radius
HC/LC bombs: 75% damage, 125% radius