Personally, I think the Val-pak or marriage mail type solutions are a waste. Sure they're cheap but you end up in a pile of 20-50 other marketers.
I've seen a few places in town partner with another non-competitor, preferably something related like dog waste clean-up. They split the cost of the direct mail and split the "ad space".
Depending on if you're purchasing your own mailing list or using the mail-houses list, and depending on quantity, I'd estimate that a post-card mailing is going to cost around .27 - .36 per piece.