My first cosplay was Stocking from Panty and Stocking with Garterbelt, which cost me about 100 dollars in all. I had an easy time with that one because the shoes were lent to me, the stockings were found for about two dollars. It wasn't the greatest cosplay I've ever done but I'm proud of the money I saved at it. Around 100-200 is pretty average in my opinion.
However, I've had costumes like Naked Snake which cost me upwards of 400 dollars and STILL aren't completed.