Last time I got a stuck valve replaced on the skyline it was free... They came out with their valve replacement tool, took off the valve cap, unscrewed the valve, screwed a new one in... Took 5 seconds, didn't even jack the car up or anything. Just replaced the 10psi of air that it dropped during the changeover.
If its the valve stem not sealing though, then thats a tyre-off job.
Also, if its the valve, then you shouldn't be losing air if you've got decent valve caps on.