magic ballista, 5d6 recharge 1 minute. In Heroes of Battle.
Check the carrying capacity of colossal animated objects, then check what 24H flight goes for, these days. 12x5x800gp, or 48000GP, it seems use-activated 2/day 12h flight effect.
64200lbs heavy load per colossal creature, presuming they're also equipped with a 36000gp +6 STR item, and are quadrupeds. (how this works I dunnoo.....) Just have multiple of those linked up to get an any-size flying fortress.
You could also check out the Stronghold Builder's Guidebook, but basically it's cost of building + cost of permanent animate object, + cost of 24 H of flight, + cost of +6 STR item, per... about 50ft by 50ft, actually, considering the 30ft face and 20ft reach. A moderately sized house, then.
3000XP for the permanency...
A Galley is 30 000 GP, and is 130ft long, 20ft wide. So, a flying ship base would be 112000GP.
The basic trick is, an animated object is a creature, and thus valid target to various boosts.