Rolling is gambling. Most of the time it loses money, and most of the time you are better off buying the module you actually want from the market. If you are going to roll anyway, here is what tends to go wrong.
Modules worth rolling
Gravid and Decayed rolls on meta modules are self-use. Selling them is hard enough that you should assume you cannot, and roll them only because you want to fly them.
Some base modules are not worth touching. A perfectly rolled T2 warp disruptor still loses to a 30km faction disruptor, so the ceiling is below what someone can just buy. Check what the alternative costs before you spend anything.
Officer modules are the expensive way to find that out. The upside is real, but a roll that lands inside the range a much cheaper faction module already covers has destroyed most of its value.
Which mutaplasmid
The rough rule is Decayed and Gravid for meta modules, Unstable for the valuable ones, so the mutaplasmid cost stays in proportion to what you are rolling.
Two things break that rule.
Sometimes the stats you are chasing are only reachable with one particular base module and mutaplasmid pairing, in which case the rule does not apply and the research does. The calculator will tell you whether the combination you have in mind can produce what you want.
And some Unstable mutaplasmids cost more than the modules they are applied to. Damage Controls and Large Cap Batteries are the usual examples. Unless you are chasing a specific high-value result, the mutaplasmid eats the margin.
