If they're buying disposable vapes, then it's likely on par with cigarettes in terms of price. That industry has been working hard on getting disposable vapes that last long enough and contain as much nicotine as possible. In my area it's next to impossible to find any disposables that contain less than 50mg (5%) nicotine, which is downright criminal.
If they're going custom then the upfront cost will be a lot more, but e-juice is super cheap. Have to replace the coils every now and then as well unless it's a refillable pod system. You can also choose whether you get freebase vs nicotine salts and how much nicotine the juice contains.
I personally believe disposable vapes are dangerous and can get a newcomer as addicted to nicotine as a pack-a-day smoker in a matter of weeks. And for smokers, it won't give you a pathway to taper off your nicotine intake.