How much to run gas brooder

Hi we were using a gas brooder suitable for 250/400 chicks in a 6ft x 8ft shed  - never had that many chicks  but oh bought  a bigger one in case we needed it later, the problem was a 24kl gas bottle was only lasting  4/5 days, I was wondering if we had it too high -  although the chicks were comfortable and thrived.

This seems a very expensive way of brooding, wondered if anyone else uses one to give me an idea if that sounds right or as I am thinking should have got a smaller brooder which would have been cheaper to  run.