After deposits on the venue, our first wedding expenditure was our Save the Date magnets.
We bought 75 mini-magnets from MagnetStreet Weddings (we were planning on sending about 80 invitations, but that includes separate invites for adult children living at home and I don't think they need separate magnets for a shared refrigerator ;). I found a coupon at RetailMeNot.com for 25% off, which brought the total cost to $80.
We were going to wait until we took professional engagement pictures to make our Save-the-Dates, but we decided that our out-of-town family needed to be warned sooner since our repeated reminders were always forgotten by the next family gathering. We sent out magnets to the most essential people right away, and to school friends (whose addresses change nearly every August) in September (six months notice was enough time, we thought, to ask our friends to reserve their spring breaks).
Ordering our Save-the-Dates was incredibly exciting. It was the first thing that made the wedding feel "real" to me.
Was there a specific purchase you made that made wedding planning feel real to you?