Well, I think first of all it all depends on what kind of build your husband is, but finding a nice suit for $300-400 (even on sale might be difficult). I used to work at Barneys New York and the best deal I ever got was a Shipley Halmos Wool blend navy suit I bought and that was still (with my discount $300). With that being said... if we stretched the budget a couple of hundred dollars... say to $600, you might have a few options - although I dont' know how your husband is built, but I'm just going to assume that he is in shape and doesn't have ridiculously long arms or any other attributes that may make him difficult to fit without going bespoke.
1. J Crew - I think someone suggested this before, but J Crew is making some nice stuff nowadays, especially their Ludlow stuff. Most of their suits are Italian fabrics, and I have friends that work high-end retail like at LANVIN that alter it a little bit and they pull it off. The fit is relatively nice (on the slimmer side compared to say a $300-$500 suit you might find at Macy's or whatever) and made for the "average" american guy. I think you can get a nice suit for $600 or so and cheaper on sale.
2. Theory - If you have a Theory outlet near you, they have some great deals and you might be able to get a suit for under $500. I've even seen the classic black Theory suits on sale like additional 35% off at the outlet for around $500. It's a great fit (probably won't need a whole lot of alterations) and although it's made overseas, a good bang for the bug. It's a nice slim, modern fitting suit without having to empty the bank account. A lot of younger people especially like this suit, and it's definatley a nice entry-level suit that's worth a look. Excellent fit for the price IMO.
3. Barneys Co-Op - a little bit on the boxier side for me (but I also am very skinny and like my things fitted - sometimes I think Theory is even a little bit boxy and it's made pretty slim), but decent quality suits. Decent fabric, decent contrustion. Trust me it's not a Brioni, but it'll probably do the job. Last time I remember they were doing the double pockets kind of like Paul Smith does and the suits were relatively short and decent fit. Spend 10 bucks on a hem, maybe 10 on the sleeves and you're all set to go. They retail around $395 for the jacket and $190 for the trousers, but Barneys should be doing their sale soon so you can probably get it within your budget on sale.
4. WARNING: I'm just throwing it out there... H&M... OK... so the only thing I buy from H&M are t-shirts, but worst case scenario, H&M doesn't make horrible fitting suits. IMO, the quality definately is far below average, but with the right tailor and the right alterations, you can make a $200 suit from H&M look pretty damn good (granted the fabric sucks but in terms of FIT which is the #1 thing when it comes to a suit, you can make it work). This is a long time ago, but I bought a cheap-o suit from H&M for like less than $200 bucks, altereted the LIFE out of it, and it looked good. The lining like ripped the second time I wore it but people were asking me if it was Dior because I had the lapels brought in, etc. etc. - - I did spend close to $200 on alterations though, and that was with a discount and pretty much had the suit taken apart and relined, and alterted every inch of it. Regardless, I couldn't afford a Dior back then so what was I supposed to do??
Well, I hope that bit of information helps, I didn't have too much to go on, but the sales are right around the corner and you should be able to get something nice in the $400-$600 range. Hopefully you can find something that fits nice, looks nice and that he can wear for the future.
Cheers and if you have any more questions feel free to ask.
