It is quite expensive isn't it! Sticking to solid for back panels would make it cheaper, or using some mesh (you can spray it white or another colour to make it look nicer. Ebay is worth a try and you could also see if anywhere locally deals in it - if you can visit face to face you might get 'offcuts' cheaper. Places that cater to gardens/greenhouses often sell panels, and thinner stuff will be cheaper - you can get 1mm stuff which is very flexible but much cheaper might might be ok if the edges are protected.