As said, Norway never gets the kinds of crushing crowds you'll get at Versailles or the Vatican Museums. And unless you have an unusual reason for visiting, most people want to focus on Norway's outdoor beauty. So, if at all possible, go in July.
And when you say "the prices are lower" in September, do you just mean airfares, or have you also looked at hotel rates? I haven't been there since 2003, but I remember Rick emphasizing that hotels were cheaper in summer, when they didn't get business travelers.
Do pick up Rick Steves Snapshot: Norway (or Rick Steves Scandinavia if you're seeing other places in the region). He has various money-saving tips, which are particularly necessary for this very expensive country.