The total budget is the most important factor. High margins are nice, but 50% on $10000 is alot more in total than 500% on $100. The more you can spend, the less you have to look for extreme margins to make good money. Adwords isn't magic, you don't have to be very clever to understand how the game has to be played, all you need is deep pockets and something scalable in volume.
It really ranges depending on the keywords you are trying to rank for. I have one client that spends $500 per day while some others spend that amount in one month. Use the Google Keyword Tool to find out what your keywords typically cost.