On my '98 GS Mk 2 the battery is a CR 1616 lithium button cell. The trade price of this is £1.20 ex V.A.T. In my experience they should be replaced (on both keys) each year. The range is pretty good on a new battery- around 50 metres or so.
If the dealers charge £6 - £10 each time, that's a nice margin albeit a small amount on money. It's only a two-minute job.
Is there no way the Club could hold stocks as an additional service for members? The cell type may vary according to the model and age of the vehicle. Steve, over to you!