the dremel high speed cutting bits i pictured around around 3-5 bucks a piece and is what i use for aluminum. these carbide bits (what i use for cast iron) are 7-8 bucks a piece.
total investment = ~$25. if you can't afford realllly nice carbide bits, do yourself a favor and at least get those and the 2 aluminum ones. working with the right tools makes a world of difference.