The way I see it, it's GW's fault for charging so much for mediocre models. I say rock up with your mantics and throw down a proverbial gaming gauntlet. Probably with the hand still inside.
In GW's defense (I know, I know), they don't actually charge that for their zombies, at least not by their own standards. Mantics are still cheaper, but it's not by as extreme a difference as, say, for the gouls or vampire knights.
I'm kind of an outlier in not being a huge fan of the mantic zombies - they're a bit large and kind of gangle about awkwardly over their bases. Not that I like the GW ones better, but I don't really have a preference. For new players, I'd recommend mantic, but I don't feel any desire to make the switch myself, given the GW zombies I already own.
The ghouls is another story - GW ghouls are neat, but their hunched over pose, reaching arms, and poking out elbows makes them miserable to rank up, while mantic zombies look good, rank up well, and aren't shoving their faces up the butts of the troopers in front of them. I've already got a bunch of GW ghouls, but I
am tempted to replace them with mantic alternatives.
The rest though, skeletons, wights, wraiths, vampires, cavalry, characters, etc... I'm sorry, but the mantic models just do not work for me. Yes the price difference is huge, but IMO the quality of what you get keeps pace with it (with some outliers like blood knights, but even there I wouldn't use GW or Mantic models for those). Not that I'd have any objection to playing against an all-mantic army, but outside of the ghouls and skeletons, I won't be going that route myself.
That said, I do try to support Mantic as a company and talk them up at the local store. A lot of their undead range isn't quite where I would want it to be yet, but I'd certainly love to see what they could accomplish with a few more years of production experience under their belts.