Even with RMU, I think you need to be careful to compare the costs you set to other professions, and build the profession around a solid profession concept, not merely around someone's intended character. Otherwise, it's easy to assign absurdly high costs to skills you never intended to purchase anyway, which will result in imbalance relative to the existing professions.
My method for RMFRP uses aptitude steps, like RMU, but with the steps dependent on the category. Irregular Realms gives more structure, but the cost adjustments will give somewhat odd costs (since they are fixed DP modifiers rather than steps). Arguably there is also room to represent the urban vs wilderness focus that is the main differentiating factor between some professions, which is not represented there. Still, if you want to let players design their own professions, the structure of Irregular Realms is a much better approach. If it's you as GM trying to balance a profession you've created to fill a gap in your setting, the other methods will give you a little more freedom and the costs may end up a bit more standardized.