Okay, well... why I asked is because my ideas incorporate trading goods or services for cakes/photography.
If one of you does something or creates something, or works in a kind of service that could be traded, I would suggest this highly. I have traded crocheted hats to a friend for family photos. Stuff like that!
For food, in autumn, since pumpkins and apples are in season, you could center much of the menu around "harvest" foods. You could have a couple of roasted turkeys (still early enough that the prices aren't Turkey-day prices) and serve them with an apple-cranberry-pecan stuffing. You can have pumpkin pies and apple tortes as desserts. For drinks, the Midwest is rife with microbreweries, you can easily get a good price on harvest beer. Or, you can skip the drinking, although that tends to be an unpopular choice amongst most wedding guests. You can ask her bridesmaids and groomsmen to each purchase a case or two of beer, as the wedding gift. They spend $30-40 on beer and you/your sister/parents don't have to spend $300-400 on booze. You can easily go to Sam's Club and purchase several 1000 mL bottles of New Amsterdam vodka and mix it with an apple or pear or cranberry drink and make a tasty "harvest" cocktail.
For color schemes... you can do red and gold. This dress
comes in a color called "apple" and since it will, likely, be chilly in the fall you can combine it with a gold wrap like this in "Sunbeam"
. You can also do oranges, greens, deeper yellows, and even brown can be very elegant when paired with nice accent colors.
For decor, you can get glass vases from a dollar store, get long sticks from the yard (or a local park with a lot of trees or a forest) and spray paint them whatever color you like. You can even get scrapbook paper and make your own hodgepodge to cover the vases. Fill the bottom with some sand and stick the sticks in artistically.
You can also scour the internet and local craft stores for clearance fabric to make table cloths with.
Aside from the food, you can easily get away with spending less than $300-400 for all this stuff. Food will likely cost you another $300+ easily. If you can get someone to trade/donate a cake and photography, you get away with a wedding for under $1000. You can search Goodwills in the nice parts of town for wedding dresses that might only need minor altering. OR... you can go to David's and get a dress for under $300 if you shop sale/clearance racks.