What's your limit on what you would pay for an automobile? Or a TV? Or a steak dinner? It depends on the particular automobile or TV or dinner.
Now, $22 for 30ml is 73 cents/ml. That's not cheap juice, but if it's really good, it could well be worth it. There are top-end juices that go for about $1/ml; these are worth it to some people. (There are also fairly crappy juices that also go for $1/ml that aren't worth it to anyone!)
There are lots of places that sell decent juice at around 25 cents/ml, and some places that sell decent juice around 10 cents/ml. I've bought juices (some pretty darned good) on sale for 7 cents/ml.
A 100ml bottle of Deadly Sin will cost about 35 cents/ml. For me that is a fantastic deal, but ONLY because I know it's a juice I really like!
It's not about the cost, it's really about the value, to you.