I've read about how MMM makes some good money on his blog. Do any of you? Is it a good source of income? How many readers do you need to make "x" amount?
If the formula was that easy then everybody would be doing it... oh wait.
The revenue question is a function of your topic as well as your blog's readers. If you're running Google AdSense then the advertisers are bidding on your blog's keywords, and some keywords are more valuable than others. Other AdSense ads pay per reader impression, and those are measured in pennies per thousands of impressions. If you're running credit-card ads (or other affiliate products like blog hosting) on your blog then you're paid when people sign up for the product. If you're linking affiliate codes to Amazon then you get a small commission (4%-8%) on the product you're linking to, but the affiliate code will "stick" to that reader for up to 24 hours. If they buy a Ferrari on Amazon right after they buy your linked product, then that commission goes to you too. If you run a podcast or a video then you can have a sponsor's ads in the content, and that earns money every time someone downloads the content-- or it's paid as a flat fee.
If you're selling other products (like an eBook or a piece of software or consulting services) then it depends on the personal relationship with your readers, what value they get from your blog, and whether they trust you enough to click the "Buy" button.
The key to making money from a blog is to write stuff that people want to read. If you're selling a product through that blog then you're writing about how people would use the product to improve their lives. You'd also need to do it in a way that would get you on the first page or two of Google search results for your keywords: a fresh perspective, a noteworthy writing style, links to your posts from huge blogs like Huffington Post or Yahoo!, writing for 1-2 years, or inventing keywords like "badassity".
To answer your explicit question: After I'd been blogging about military personal finance for two years, I started up AdSense. The first month was $133 and the lowest month was $98. By the time I sold the blog (eight months later) I was earning $250/month from a combination of AdSense, Amazon Associates, and individual ads. The new owner has probably quadrupled that.
A cousin blogs about health eating (mostly paleo), with lots of recipes and photos. She's been doing it for two years, she's funny and snarky and enthusiastic, and she has a compelling backstory. She has an enthusiastic audience of probably 2000-3000 daily visitors. She had her first $1000 month this year.