I have been collecting RCS and getting the empty Soul of the Crafter item. This item allows you to change specializations. This is allowing me to craft all of my i190/i195 gear myself rather than making some and buying the rest from someone else.

That's the "main" use for RCS right now that I can think of. Although T4 Materia are another viable option.

As for opportunity cost, that depends upon how quickly your stuff moves. If you think that 4 adamantite nuggets, 2 titanium ingots, etc. for an Adamantite greatsword are collectively worth more than one T4 crafting Materia, you still run afoul of actually trying to get those materials sold (with the MB penalty "tax") and then finding that Materia on the board. Some of those should sell rather well (like the Adamantite). Others, not so much.