I don't launch a SIB from there, just the 6.5m. I have seen SIBs launched from there tho. I do not know the single launch costs; there does not seem to be any thing on the web either. My annual pass was £140 last year. At a guess the single launch price will be in the range £10 - £15, but do not hold me to it!
I cannot think of any suitable beach launching sites in the area, the only alternative that springs to mind is Keyhaven, there is a slip opposite the yacht club.
Dust creation specialist