They're cheap because they're sheet bronze cymbals, not cast bronze. You get what you pay for.
That a cymbal was formed out of a sheet with others, rather than an ingot (later to be made into a "mini-sheet") in the case of a cast bronze cymbal, does not make it of cheap (which I read to mean "poorer") quality, regardless of workmanship after the blank was fabricated. It is merely an issue of scale—that is to say, ten cymbals made at once versus one, with sheet and cast bronze respectively.
Now, it could be accurately said that because of the alloy used (~8% tin in sheet bronze cymbals v. ~20% tin in cast bronze) that a sheet bronze cymbal is economically cheaper due to the cost of tin. Some drummers also prefer the tonal characteristics of one alloy over another, which also justifies the need for discrimination.
The reason that ZXT cymbals are cheaper is that not a lot of effort goes into shaping and lathing them, as well as the mere price of the alloy as mentioned.
Now, if I may speculate, I understand that sheet bronze (of which the ZXTs are made) is not as brittle as cast bronze, which is why it's the metal used for sheet cymbals in the first place. In effect, this would render the ZXT cymbals stronger than some other cymbals and therefore able to take more active playing.
