We sent out Save the Dates, they cost us about $150 with postage, return address labels (vistaprint), info sheet, and a magnet (also vistaprint, soo cute).
People got really excited, my almost hubby wrote a witty letter with our intentions(we added a picture of a palm tree and color photocopied them on white paper) and even though we didn't have a specific date or resort, we gave our wedding webites and have updated them from there.
Our friends and family loved it. It got them excited, they have thanked us for the advanced notice, and it drummed up support for the trip. The magnet will hopefully keep reminding them everytime they go to their fridge. So if you can afford it, I would do it, and it doesn't need to be expensive or elaborate.