Few things here:
- As of now, Stormblood-endgame-crafting doesnt require pentamelded gear, even less so pentamelded with Vs and VIs.
- Personally, I didnt buy a single materia on the market board to overmeld my gear - I'm aware that that comes down to luck to a certain degree, but in my experience its a lot more related to careful planning. And by that I mean: We had MONTHS of weekly deliviers between the new crafter-sets and little to spend our scrips on - except for materia, which made stocking up on it extremly easy. Weekly delivieres alone should be enough to buy about 18 grade Vs and 4-6 grade VI materia. I can only encourage you to stock up on materia now by doing your weekly deliviers, so come Shadowbringers you'll have less trouble. Not a single gil went into my crafting gear.
- Little extra-tip: Encourage friends and FC-mates who arent intrested in becoming mastercrafters to at least do the quick weekly delivieries. Offer to provide them with a macro that they just need to hit to craft those things. Have them buy the materia for you - and offer to craft their gear and maybe food and potions free of charge!

You should also consider if overmelding your gear right now is really worth it - granted, we still have a few months of Stormblood, but its unlikely that we'll get any new major recipes and the demand for crafted gear will only decline (since there are more and more alternatives), as will the demand for food and potions (since less and less people still raid AND require those items to do so).
To get a headstart in Shadowbringers, pentamelded gear wont really be worth it and you'll throw it in the bin once you hit level 75 (at least thats my guess) - better to stockpile this materia now and become a pentamelded mastercrafter in 5.0!

And while I can agree that percentages below 10% are horrible, I have to point out that at least for those last sets they werent required and that it has become very easy to get the needed materia - specially when you plan ahead.