My horse persistently eats his bedding, please help!?

my horse consistently eats his straw. he's now even started eating it before his hay, he'll eat the whole of his bed, then his hay, he's now even choosing it over his dinner! his other problem is he's so so messy, and shavings being nearly eight pounds per bale, is going to cost ridiculous amounts of money, as i will need to put shavings in near enough everyday as he is so messy. im going to get some rubber matting to cushion the floor, and ive tried spraying disinfectant on the bed aswell, and at first he didnt eat it, but then he just started eating it again regardless. has anyone got any other alternative bedding options, on the cheaper side?! thankyou