I haven't built a ton of backdrops, but I always used latex paint, mixed by the home center, on the ones that I did. A lot of modelers that I know do the same, because, as you found, the cost of the acrylics in the tube would become astronomical. Plus, you're not going to be constantly mixing new batches and risk a mis-match. Go buy a cheap gallon of your favorite sky colored paint, perhaps blend it with white or light gray from the bottom up to create haze in the distance, and post pics of your result.
DFF