Errant makes me a bit of money. Not enough to live on, but some money on the side, so to break things down...
Ad revenue: Not worth focusing on as your main income. At 2800 odd subs and good engagement compared to comics of that subscriber count, Errant still makes me less than a dollar a month from ads. For this reason, I'd strongly urge people not to use sub-for-sub to rush up to 100 subs as fast as possible. You need people actually reading your comic to generate views and so get revenue, and even then, it won't be a lot until you're up in the tens of thousands.
Donations: This is where I've made most of Errant's income. I've had regular donations and I take part in Inksgiving as a visible member of the community, so I've made like $800 this way.
Patreon: I run a patreon where readers get pages a week early and access to a private discord community. It makes me about £30 GBP a month (I think that's around $45 USD). If you want to make money off a webcomic, having a Patreon is a good idea, but don't expect to get a lot of patrons immediately.
Kickstarter: The Errant Book 1 Kickstarter made over a thousand pounds last autumn, so after paying for all the printing and stretch rewards and stuff, I was left with a few hundred quid profit. Not bad! Plus I make more from selling them at events too.
Merch: I make a little off merch, mostly stickers at events. It's not worth focusing on too much until you have thousands of readers. Merch that looks cool, cute or is funny without needing knowledge of the comic tends to sell best.