How about using the internet? Let's say you are a good digital artist and you already own programs like Painer and Photoshop, or feel comfortable using less 'bulked up' programs (like OpenCanvas) or freeware that will only charge you a small fee when used for business purposes. You could invest about $100 in a program, and a few bucks (let's say 50... and that's overdoing it just to be sure) for hosting.
Startup expenses: 150 dollars, plus 50 for unforseen fees and to make a nice round number 'cuz I'm lazy like that.

ammounts to 200 dollars.
Everything after is free, since the internet isn't going to charge you for painting a picture and selling that.
Once you have enough images you think you can sell, you'll still have 1300 dollars to spend on a service that makes products of your art (posters, mugs, magnets, T-shirts, cards...), buy advertising space on Google or other sites, repairs to your computer...
Same goes for writing books: think you got what it takes to be an author? Spend a few bucks on some good how-to books and lessons (if you even need those), write a book and try to sell it! An agent will ask for a commision, but only if the book is indeed sold and they then take a share of the profit (never EVER pay an agent! I can't stress this enough! If an agent wants you to pay fees, it's a scam!). Your total expenses? Educational material (optional), a computer (which you probably already have, unless you're posting from a library), and wordprocessing software (which can also be found for free!).
You don't need a big wad of cash to start making passive income