I suppose the answer is relative to the pocket depths of those shooting. I personally do not shoot a lot of driven birds, but £1200 does not sound over the top.
Whilst a fair sum of the hard earned stuff I would be willing to part with it for 8 days of good sport in the company of friends.
Ultimately driven shooting is an exspensive pass time, primarily because of the costs you mentioned. If your prices are comparable to other local shoots I don't think you are too steep!