When you equate the cost per shot film cost to roll film you are comparing apples to oranges. Personally, I might shoot 6 -12 images on a BIG day with my 8x10. If you are bent on 8x10 I don't think you are going to save very much by investing in 4x5 first, (the basics learning curve is short and brutal for either) however there are many instances where 4x5 is much more convenient to use, and over time, yes of course it will be cheaper, but then again, your negs will be 4x5. As far as negative size making a difference, personally I have never ever seen anything that approaches, let alone rivals the qualities of a good contact print, and that's what 8x10 and larger is mostly about, contact printing, which is, in my opinion, still the zenith of photographic endeavor.