Usually with kits they're cheaper than buying the fabric individually. Plus if you need 1/4 yard of something, the kit would include it but most websites require a 1/2 or 1 yd minimum cut, so you end up buying way more fabric than if you just buy the kit... from experience
The kits just seem expensive since they're upfront about costing $50. The fabric is deceptive since each yard is only $8 or whatever, and your cart has the final total