I've made a budget cube with cards from M13 and before below $0.5. I started by looking which are the top cards in cubetutor for those sets. Here are the most played cards from M13 and before below $1 sorted by cube appearance:
I only have to fix the number of cards for each guild, so there are 80 multicolor cards for a total of 540 cards. I would also like to include the most iconic magic cards under $0.5 like serra angel. Which ones come to mind for you?