Yes, Epson ink is fairly expensive, and it's easy to look at the cost of an entire set of cartridges and consider purchasing third party due to the savings.
However, supplies in this industry (should) make up a small percentage of your gross. I'm going to off the top of my head ballpark this, but ink probably runs you between $0.33 - $0.60 per square foot. Take a look at what prices you charge and figure out your per square foot gross. It's (hopefully) many orders of magnitude higher than the ink cost, like $8 per square foot likely as a minimum, probably higher.
If you cut your ink cost in half, yes, you've saved a lot for a set of inks. But was saving $0.15 - $0.30 per square foot that you're reselling for so much more worth it?
I haven't looked into it for a while, but I don't think there have been any reputable lightfastness testing done on third party color inks for the semi recent or current Epson Stylus'. That should likely be a big concern for you, unless you're only printing things intended for short term display. (If you know of any, I'd be interested in looking at it, even though I probably wouldn't consider the switch.)
You often have to consider the chance of the ink doing harm to your printer. Although I think the risk is overplayed by manufacturers to keep their market share, you occasionally do hear about someone's printer having issues that are due to using a third party ink.
Furthermore, I don't know for sure, but I would imagine the stock profiles for Epson ink might not be very accurate for third party inks. Perhaps color management isn't of extreme importance to you, or perhaps you custom profile everything anyway. Or, who knows, perhaps third party ink suppliers create profiles for some media types -- but you'd still have less profiles available when you start looking at third party media. But, otherwise, it's introducing one more variable to increase how many reprints you have to do -- which is the most expensive resource of your time.
