Save the dates for weddings are important; your guests need to plan ahead, but it is a fact of life that most will end up in the garbage. I know I have received some expensive save the dates like magnets and elaborate paper invitations.
To keep the cost down, make your own postcards. Use a picture of something meaningful, like the place where he proposed, and combine it with a vintagey Martha Stewart postcard template for the back. Find a local, inexpensive printing company for production. You should be able to get a good price on it, since you created the design yourself.
Photo by Shutterfly via Style Me Pretty