Charlie,
We are at the moment in a part of the rapidly escalating Colt DA inflationary curve that says that good of an example of the DS (98%) would be a very good deal at $ 500.00. Perhaps a bit more depending on the area of the country. You won't be stealing them at that price but many will go for more right now.
The Cobras will generally command more than an Agent (and be on a relatively even scale with the DS) as they were a bit "higher finish". On the other hand Agents are found less frequently than Cobras and certainly less frequently than DS. The collectors trying to fill out the Colt set may spend a bit more than expected on the Agent but I would want it at a lesser price than the DS. There are "snake gun" collectors among these Colt-o-holics that are trying to posses an example of every Colt with a snake name. The Agent (and to a lesser degree the iconic DS) lack that additional inflationary "push".
Hope that helps and any estimate with good Colt DAs is a very perishable commodity. They are going up that rapidly, and they won't get cheaper with time.