Congratulations on your engagement!
When my wife and i married we wanted friends and family around us, but we were determined not to spend a lot, so here are a few of our ideas:
We had our ceremony and reception in my Mom's lovely backyard by the pool. Maybe you know someone with a nice backyard you can use for free? September is a good time for an outside wedding. You can rent chairs for a small fee, or ask friends and neighbors to borrow a few.
We had a small plain cake made at our nearby grocery store, and went to a cake decorating store for a topper. We had cupcakes made up in our colors, and there you go for maybe $50 total.
If you have a 99cent store/dollar store, keep your eyes open for deals on matching paper plates, napkins, cups, plastic forks...etc. You can also find seasonal decorations there for cheap...we were married in July, so we bought summery things and tikki torches for around the pool.
If you or your friends know how, do some music mixes, and have one of them help you with the music...maybe cost you a six-pack or it could be their gift?
If you plan to have booze, do a spiked punch for the adults and a regular for the kids? Cheaper than beer and booze set-ups.
To go with our summer theme, we bought great silk flowers at the local craft store and made all of the flower bouquets and coursages. I still have ours!
Printed Invitations are pretty pricy, so make them yourselves, or buys some plain and have a crafty friend do calligraphy for you as their gift.
So many ideas, lol
Make lists of things you really can't live without at your wedding, and go from there. We found we really couldn't live without our family and friends there...everything else was icing on the cake!