Sagitta can be had for ~140m, heroism crystals for ~70m, so it costs about 210m and 5 minutes of your time.
Vereth is 91m of hmp, 42m of riftcinder, 58m of riftborn boulders, 130m of detritus, and 50m for clear(if you need that), so 321 or 371m. It also takes about 22 hours nm camping, 7 hours vnming, and best case scenario another 12 hours of abyssea, so throw in 41 hours of play.
If you make around a mil an hour and have to buy the emp clear, it's potentially twice as much. Realistically, it's less.
That said, I made Sagitta and augmented them for my MNKs. I know Vereth is better, but I also know I'm unlikely to make the 41 hours of free time to make them(really, moree, since each mnk would need the abyssea stage). I don't regret my decision, I have a functional weapon with some nice side benefits(DT and HP).
If you plan on eventually making Verethragna, I would agree that it's rather foolish to go for Su5 now. But, if you want to have a functional MNK and don't feel you'll make Verethragna, Sagitta is a solid option that will cost you less gil and no time.