Wait, if the new UUM uses both a fabricator and blueprint-based assembler, can't you balance the cost of things like grass, lapis, and even diamond with relatively little UUM but expensive to craft and hard to find blueprints and energy-demanding assembling while making the metals (iridium included) be based on larger quantities of UUM?
from the in-game logic,it at least just to represent the difference between materials that are complicated, having complex chemical structure and energetic molecular bonds (grass, logs, rubber, gunpowder) the materials that are heavy (stone, obsidian, gravel) and the various mixes (Diamond being extremely complicated, iridium being extremely heavy, coal being slightly heavy but very complicated, clay being slightly complicated and a bit heavy, etc)
Gameplay-wise, this also follows a progression: once you start making UUM, you can make things that require small amounts of liquid, have easy-to-get templates, and easy assembly (things like stone, sand, dirt), you can then get more complicated blueprints (giving you things like clay, flint, glass, grass, logs, rubber, water), get even more UUM and some blueprints (getting you obsidian, copper, tin), some more blueprints (for coal, redstone, lava [needs to require a lot of energy to assemble]), a lot more UUM (for iron, gold) even more complex blueprint for diamond (even in a vanilla world, if you get a fortune 3 pickaxe, after a while you can get enough diamonds to last for a lifetime), and a truly ludicrous amounts of UUM to get iridium.
(this is, of course, assuming blueprints have to be found and further crafted to be used in an assembler, assuming it's a machine that require blueprints, energy, and liquid UUM to craft items)