That's not a great strategy to just continue to pump $15 into fiverr hoping to eventually get something good. Does that happen when you get to $500 or $5000. You don't know when you start so you could just waste your money.
But, for rapid development of prototype, or early stage products, I could see a few $15 injections going a much farther way than abandoning a project and having dropped $299 or even more into a 99Designs (or 'higher-end') logo[0].
I would always double back and get a more professional looking design that is also coherent with the Web App look & feel or product page look & feel.
