I only use fly repellent when we're going out for our walks inhand/school work or any ridden work, because then he doesn't have a rug on.
So I don't go through a vast amount really, it's his sheath (and ears, but he always has a fly veil or mask on for those) that he needs the spray around mostly.
I have the Weatherbeeta Fly Sheild. Actually, I have one and a half.
It was about £60 (each) initially. I bought a second because I liked the first so much; it is fabric mesh rather than plasticy mesh, so it drapes around a lot better than most rugs. Then one of the other ponies in the field I think, was bullying Daffy, and once a hole appeared, every day I'd be mending one rug and luckily, putting the other on if I didn't have time to do the repairs.
It got to the point where one of them had so many holes (and no fabric to patch them) rather than tears, that I use the original rug to patch the newer one if necessary. I've only had to repair the newer one twice (repairing old repairs!) since moving yards, where he's on individual turnout, which makes me confident that other horses were wrecking his rugs; he's never damaged any other rug he owns.