We bought ours at AC Moore and I made the pockets. With coupons and their constant "Madness" sales the total price for the cardstock for 150 invites was less than $20! That includes one color for pockets and 2 different colors for the inside inserts and actual "you are invited" portion.
Just another tip. We bought our envelopes at mygatsby.com in their clearance section, also right around $20. They don't have a huge selection of colors, but we ended up with gold and it offset our colors nicely.