I bought print-at-home stationery at a Papyrus store (it was on sale!) and designed the nightie and text on Photoshop. I used a picture from the Web to draw the outline of the nightie. Then I bought little stick-on bows at Michael's to dress the invites up a bit. The total cost was $30.
Holy shout-out, Currer. Two noodles up for you!