You farm enhance mats from one of three 15 energy desire vortexes, depending on which color you want. Some enhancements are super cheap, I enhanced Agrias' full break once and the material cost was insignificant. There are two caps for enhancing the major abilities though, T4 stones and T5. T4 stones are a soft cap, they drop in the vortexes and are extremely rare. T5 stones are a hard cap, you get 2 of each color per month from an event, this month it was the Ayaka/Gouken event. You could get another one of each color from the Kingsglaive event, but each one was 100k raid currency so not realistic for anyone but whales. I actually have enough red T4s to +2 holy explosion if I wanted, but I only have half the T5s I need. I'm waiting to see how things shake out with my roster before I commit to any enhancements, and I don't want to invest too much more energy into farming stones until I have my raven's mantle.
And yes, it turns out the gil cost is largely irrelevant. I have just over 10m gil on my GL account. I've been playing on JP for a month now and yesterday I passed up my GL account in gil.