I looked at levitation, reverse gravity and telekinesis and getting some that can be at a continual cost for a high level spell was ridiculous (millions gp).
Two items that each create a permanent gusts of wind (60ft surface area - perfect for a schooner's shaped sails which could easily cover this ) which is steady wind of considerable force (50mph)
=> 100,000, this is 1/2 the price but more demanding as these wings would need to be "sailed" - looking a wing area would be disastrous (ie not much has to go wrong for a wing or sail to have issues => Hurting a keel is a lot harder)
The cheaper but more demanding version I like for it is literally a flying ship - ( it is creating its own wings that are being sailed / used to move it up and down when less of a surface area of the sail covers it ). It has a primal grace about sailing, to me its more romantic and would have a true feel about being in a flying ship - not a floating - air-baloony one.
The flying ship would need people always using ropes etc to constantly be adjusting mail sails and wing sails, so it would be a hive of activity and feel more alive (and have more character I think).
Still 200,000 for one that is solid less risky ride (You have me thinking mothership ideas lol :) is interesting to know.
Strange it didn't say how big a ship the keel could manage... on a Galley ? or Single sail boat ?"
(Galley is worth about 30,000)
Still it could be something that could be plot driven.
Not sure if the sell a cloud keel anywhere near me nor have i heard of it, also if they do sell them ? Or how many would there be?
"Cloud Keel: An invention of djinn on the Elemental
Plane of Air, the cloud keel gives its vessel the ability
to fly at a speed of 40 feet (clumsy). If the vessel is
wind-powered, it can still use its sails if they provide
faster movement than the keel itself.
Caster Level: 17th; Prerequisites: Craft Wondrous Item,
fly, wish; Market Price: 200,000 gp; Weight: 1,000 lb."
That means a wish is needed to cast that spell. So its unlikely there are many.
The reason its important is how many ships have these heels ? Is it likely merchants would know of 3 of them or more ?
That heel's price the same cost of having 2 gusts-of-wind flying schooners (on craft which wouldn't be as safe)
Gust of wind apparently can be made permenant ... http://www.d20srd.org/srd/spells/permanency.htm
I dont understand the rules to work out how much it would cost to have other cast it - for it gives the cost in xp for the caster.
If two of them being cast by the permancy spell is cheaper than 100 000 in total. Would be the cheapest option.