Weddings are generally the ultimate in meet-your-hero-letdown. Everything is fussed about in minute detail, but then it's all over, and you're left thinking 'what was all that for'. The memories of the day and what comes after has zero to do with wedding stationery. I can't even remember what color ours was. I'd rather spend the money and fly in a distant relative or old friend than spend big money on invite printing. Your guests will appreciate and remember hours spent refining a speech over hours fussing about invites and place settings. There is definitely diminishing returns once you get over pre-printed and hand-written invites.
Getting back on-topic : I think you'll do well with this and the wedding industry is over-priced, but a lot of it has to do with ego-management. If I were a photographer or florist, I'd add 20% to the price to put up with all the 'it's got to be perfect' crap that goes with it. Self-serve printing removes all that, so is justified with a lower margin. As long as it looks nice, people will pay the money. You'll be able to pick the cheap paper without someone frowning down your nose saying 'if you really want it to be special...'