There are heaps of ways to save money.
I have found the key is to avoid anything that is for weddings. For instance, if you want some paper to make invitations and if you buy them on a wedding invite store, they will charge a lot. Whereas if you just look for a paper discounter (for offices etc) you can get the same thing cheaper.
The wedding industry really pisses me off. And bridal magazines - I vowed to never buy another one again as I was getting sucked in to all the things they manipulate you into thinking that you have to have.
I made our save the date cards for about $30 - found a discount online printer that deals with small businesses etc and designed them (so cute, looked like little postcards) and we got 100 of them for that price. They even threw in some free address labels and thank you cards.
The pot luck thing is a great idea - if you can get people to bring food you can save a lot of money. If you know someone who has a nice garden / house maybe you could have it there?