What budget you choose for your DSA campaigns is going to be something you'll have to figure out. I have seen that DSA's typically have a much lower reach, especially at first, so starting with a lower budget than your normal campaigns would be fine. You'll have to do trial and error, but I would think 10-20% of your main campaigns is a good place to start.
Are you using AMO, integrated with Adobe Analytics? If so, you may want to talk to your Account Manager about the server side s_kwcid integration, as tracking revenue for DSA's is impossible without it.