Likely worth a ton to a few people. Unless you're swimming in gear and pds I'd sell it; it isn't going to make a huge difference. I wouldn't be surprised to see it go for way over 30 in an auction given the rarity of 30 attacks and of grants in particular.
Here's the thing: A person will never get all Lv. 30 attack techniques. Not with 50 thousand hours of gameplay could this be done. Therefore, no matter how many you get, you'll always have to wear a /Technique unit if you want to see all of them at 30. So there would have to be some huge aesthetic value to see "30" on that ONE technique without the /Technique unit because, let's face it; the performance doesn't justify the price.