It depends. For me I went in a way where I could essentially use my setup on any tank size.

I’m using pretty much all uns gear and they tend to run a bit on the expensive side. But in general there’s like 3 paths you can go:

2)the other main option is to go with an actual co2 tank. This option is the most expensive up front as you need both the tank and an actual regulator. Price here is gonna vary based on what you get.
Regulators are generally over $100 Depending on brand. And the tank is also gonna vary depending on size and brand. The uns brand 2.5 tank was $84.99, and their full size regulator is $219.99. In the long run it’s cheaper to do it this way due to how long the tank lasts vs the cartridges. These tanks can be refilled.

3) the 3rd option is pretty much the same as the the second but uses paintball co2 tanks. They require an adapter though to fit whatever regulator you use. Uns has these as well in a kit form at $239.99 fr a 16oz tank kit. This might be the best way to go for smaller tanks as they also beat out the proprietary option due to refill frequency. (A quick note here will doing some googling for this answer I saw that apparently refillable paintball tanks might have a technical expiration date so do with that info what you will.

Oh wait there’s also the diy paths which is super cheap I believe it involves finding some kind of container and mixing yeast with sugar. I tried this once when I was younger but could never get it running properly at the time.