I'm an heirloom sewer and a beginning quilter so christening gowns are close to my heart. You can use heirloom techniques and not have a tremendous amount of money tied up in the gown. Of course you can also go way over the top and have lots tied up in lace alone. Think about tucks (traditionally in 3's) or puffing which I think is lovely. Spechler-Vogel makes Imperial Batiste which is a cotton/poly blend, very nice to work with and pleats well for smocking if you're interested in that. Don't let smocking scare you, it's really quite easy especially if you're doing a geometric design. If smocking, you might also think about incorporating pearls into the design which has a very elegant look to it. A simple bishop dress can be a christening gown simply by adding length to the bottom with lace, swiss insertions (like an embroidered eyelet strip), puffing or tucks.
Don't be afraid...if you can quilt, you can make the gown. Have fun and best of luck.
Marthapullen.com is a good resource