Truthfully, I’m terrible at dressing “casually”, and I also don’t recommend going too “cheap” on shoes. So, that’s a bit of a tough one.
On the low end for chukkas, you could definitely look at Clarks desert boots in the beeswax leather. My roommate has a pair and I think they’d probably do fairly well throughout the fall weather. I’m not going to tell you they’re waterproof or anything, but they’ll be decent in a light rain. For what it’s worth, I wore my sand suede desert boots in an outright downpour this summer and they’re just fine. I stuffed them with newspapers when I got home and let them dry out. Texture’s a bit different, but I’d say they’re a slight bit more comfortable now.
If you’re looking for a pair of higher-quality chukkas, then I’d consider giving Allen Edmonds a look right now. Their “Rediscover America” sale is going on and the Malvern is on sale for $276.25. Depending on your size, you might be able to score them cheaper on eBay.
I’d also give consideration to Charles Tyrwhitt’s chukkas (brown, dark brown, chocolate suede) which come in at $217 before shipping.
Speaking of Charles Tyrwhitt, give a look at their chelsea boots (black, brown). I’ve worn them with denim in the past.
I also think captoe boots look pretty good with denim — heck, any boot generally does. The Allen Edmonds Bayfield is $262.50 at Lands’ End right now (code SAVEFF with PIN 9132). Charles Tyrwhitt military captoe boots can be had for around $250 on eBay. L.L.Bean Signature captoe boots are under $200 and are Goodyear welted. If you’re willing to spend a bit more and are an eBay ninja, I’d look out for Alden Indy boots.
Hope this helps.