I haven't spent any money and barely any time on marketing. Development I did all the coding myself and eventually hired a graphic designer part time. The thing is I'm not usually on the Top 100 list (or even Top 300 for that matter), I just charge more for my apps (my price points are $3, $5, $8). That's the best kept secret of the app store. Sure the Top 100 apps make a ton of money, but there is a growing class of niche apps that can make just as much money by charging more and catering to a smaller niche audience.

My advice is to not create the next Angry Birds, but try and find a niche that is underserved and create a great app and charge as much as you can get away with. And get a great designer too, that's the best ROI I can think of.