I can't give you an exact budget, but I have some tips from my childhood growing up in a family of 5 (we were 3 girls, though) that lived on a single grad student/postdoc budget.
One way to improve wear on kids (and adults!) clothes: have "home clothes" or "play clothes." When something gets worn, stretched, embarassingly unfashionable, or torn, patch it and relegate it to home wear, as opposed to school/outside wear. So, e.g. your husband could change out of his nicer clothes into the more worn clothes after getting home so his nicer clothes stay nice longer.
If the kids lose mittens often, you could make old-school "mitten leashes." Stitch to an elastic as long as their armspan and thread through the coat sleeves. (I think I remember doing this when we were really little.)
Start a "pipeline" from someone you know with kids just older/bigger than your oldest; we had some friends that routinely gave us bags of stuff. (And another with someone younger than your youngest to give things away to so they don't clutter up the house!).
In terms of wardrobe size, we tended to stick to about 5 pairs of everyday pants, about the same number of sweater-weight tops and shorts (for summer), about 2-3x that much lower layer shirts, one each of sandals/summer shoes, sneakers, boots, light/rain repellent jacket, heavy coat. Maybe 2-3 pjs. We often had more that we acquired through hand me downs, gifts, etc, but that's about the size of when we would go shopping (this usually meant we were running out of something before laundry day). I've mostly stuck to this wardrobe size as I grew up, except I had to introduce more variations...business, athletic, etc. :(
Hope this helps a little.