How much do you pay for "THC oil"? How much is vape juice base? How much is the "flavouring" of your choice? How much is an hour or so of your time?
In my case, the base is €16 per litre, flavorings cost about €5 a bottle of 15ml and 10ml of 10% THC oil is around €70 when you can get your paws on it.
So, in my case, you're looking at at least 20ml of vape juice for not really much more than the price of that cartridge and, likely, closer to 80-100ml depending on the strength you desire, plus a little bit of your time.
I don't think I have to paint that picture much more, and I don't think you need a calculator to see that spending $72 for that cartridge means you must have a glass cock because someone's seeing you coming....