I have been asked to make a 3-tier wedding cake, enough to feed 150, and sheet cakes to make up the next 200 servings. My question is; how do I come up with a price for that? I have never been asked to do a wedding cake with sheet cakes to. Does Wilton have a chart for those so I know how much a sheet cake would feed?

http://www.bakedecoratecelebrate.com/techniques/amountbakingguide.cfm

I use this one. HTH

You could call some custom bakeries in your area and ask how much they charge for a sheet cake and how many slices there are. Then you could charge something similar to what they charge. You could do the same with the actual cake itself. Is the cake going to be buttercream or fondant? Decorated simple or ornate?

weding cake sizes http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
as far as i can see, they only have sheet cakes in party servings. http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm but hopefully, it wouldnt be too hard to figure it out so servings are the same size. and of course, sheet cakes are technically different from kitchen cakes as well, so be sure your bride knows that.
xx