From the SRD:
A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical.
If the item is one that takes up a specific space on a character, any additional power added to that item doubles the cost.
Am I reading this right? Does this mean I can have my +5 Battleaxe enchanted with the Keen ability at the cost of a +1? We had been playing it that it was the same cost as going from a +5 to a +6. Basically meaning I can add Keen for 2,000 instead of the 22,000 I thought it was going to cost me. Even if you say that it is adding to something that is takin up a specific slot then it is double the cost it would only cost 4,000 instead of 22,000.