I’ve never worked with proxies before. A lot of services charge per GB, and right now I honestly have no idea how that even works. Or maybe there are services where you pay for GB based on actual usage?

There are often minimal packages. They're also often called test packages. Free or trial periods are rare, maybe no one offers them at all. But these mini versions help you test everything before paying for a full proxy.

I assume it all depends on your activity. Basically, the same thing applies to regular internet usage. You also buy a certain number of gigabytes when you pay for mobile data, for example. For some people 10 GB is enough for a month, for others it lasts like a week and a half. There are calculators like this https://floppydata.com/proxy-traffic-calculator/, and in my opinion it is really convenient. But if you are buying proxies for the first time, I agree with the advice about starting with a minimal traffic package just to test things out.