The unique zeta abilities cost more because they're more versatile in that they can be anywhere in your squad and will always be active.
All zeta abilities cost 20 zetas, it's just 3 more omegas and 10 more mk 3 mats. If you can get 20 zetas you shouldn't really be complaining too much about 3 omega materials and 10 more purple mats.
However, Rey's zeta is pretty much useless as she's not used as a support/TM controller, she's used to whack people and whack them hard so they die. No point in Dazing a dead body. I agree they should've made it some kind of expose function or honestly just expose and daze would be alright. That way the resistance team would be able to expose through tenacity up to start the match off a little more easily against Rex/Yoda/Chirrut teams.