I think a big reason we spend so much on defense is on defense you tend to play more players. As on offense ideally you only play 1 QB all season, 1 group of 5 offensive lineman, 1 sure fire WR and TE. By contrast on D you're constantly shuffling in DLineman, maybe 1-2 more DBs on third down etc. As a result paying for quality depth more important on D vs O.