The pricing model we’ve gone with is a flat $1.00 USD for each unique Tile.
It doesn’t matter if you’ve made a super-simple one, or something extremely elaborate; its all the same price. This way you get to concentrate on making the Tile you want, rather than getting distracted by increased prices.
In any case the prices are quite low; we certainly didn’t want cost to be a barrier to using the service.
The price is ultimately up to you though; I mean you could get away with re-using the same Tiles over and over to make your Assembly and wind up paying just a few dollars. Or you may be someone who will tinker and refine to get their optimal Assembly, and that could cost substantially more.