No set amount, and it's in my "grocery/household" budget.
I'd say $50/month? But I go in spurts. Easter weekend the Sal Army was half-price on their clothes and I spent about $25 there. Last week DH needed new underwear and I bought some socks for myself and then spotted at Wal-Mart jeans that were actually my size on clearance for $3! I admit I didn't really need them but for $3 I just couldn't walk away. The kids get new clothes when they out-grow their old stuff, youngest DS gets hand-me-downs most of the time, and I don't buy clothes going into the new school year "just because" it's a new school year. We do end up buying them new sneakers every fall, but since they start the school year in shorts I don't buy any fall clothes until the weather cools down.
And I very seldom pay full price for anything.
Having said that, DH has a uniform for work so he only needs casual clothes, and neither boy is picky about what they wear.
I think I'm lucky though, because there are plenty of opportunities for me to get nice clothes cheap around here. The Salvation Army gets pretty good stuff, there is a yearly used "career clothes" sale where I work, one of the big organizations does an every-other-year rummage sale that is huge huge huge and has a ton of clothes.