Thanks for giving ERA a try!
The base product is free, and contains sample files that you can customize.
The "paid" part of ERA is the set of Attack Tables, Criticals, Spell Lists, Fumbles, Professions, Races...
You could in theory create all of your files without spending a single cent, and that is part of the idea of the product. This of course is more cumbersome and requires some technical skills, since all configuration files use XML format.
This thread is where all the "under the hood" questions are posted. You may add your own if those already there do not accomplish what you need.
It would be great if you could post there if what has already been asked is not enough for your intended customization, so that the answers can be all bundled in that thread.
Also, if you haven't downloaded any version up to now, I'd recommend trying the
latest beta version, which has some additional features not yet included in the one available through RPGNow. It may take a week until we upgrade the public one, at which time we will also add all RMFRP Cultures for those who purchased Character Law, and all Large and SuperLarge criticals for those who purchased Arms Law and Spell Law, both for RMC and RMFRP.