I managed to swap my bag of shavings/sawdust for a bag of Back to Nature. It was very expensive and I am nearly through a whole bag in just a week! Might have to re-think this again!
Switch to megazorb, sooo much cheaper. As a guide a bag lasts about 2 months to do 2 litter trays for 1 rabbit. There are a few sites to get it from, I use The Hay Experts.
Or find some cheaper wood pellet cat litter, its usually cheaper if you buy the big bags.