Actually, it may be the opposite.
Although logic would seem to dictate that smaller material would be less expensive because there's less of it, it's more likely the opposite that' s true for worthwhile, purpose grown shohin material.
Well-grown shohin material can be ALOT more expensive than lesser larger material. You have to look no further than here:
http://riverbaybonsai.com/index.html
at any of the tremendous shohin plants to see they command the same high prices good larger bonsai can command.
It's not surprising if you look at what's going on in growing decent shohin material. A rule of thumb is that the smaller the bonsai, the more expertise required. You have less of everything to work with--leaves, branches, soil, pot. That leaves less room for error and less room for "fudging" branch placement, roots, etc.
All this doesn't mean you can't buy a cheaper smaller plant to work on. It means you will have to work extremely hard to find worthwhile material that "works" visually as bonsai. It will also take more skill to develop it as bonsai. Larger bonsai are mostly for lazy people who don't have the skill to work on small trees, which is why all my trees are over 24" tall and have pots as big as washtubs.