How many dollars can fit onto one pallet?
A large leverage wrecking bar, claw hammer, spreader clamp and sweat and patience. They are certainly made to resist coming apart in a fashion fit for reuse.
If your doing it cheap and fast. Take a skill saw, cut the inside edge right next to the 2by, then take a nail puller and hammer and pull the nails out of the center. Now you have boards on the top and bottom. If your looking at keeping the sides... then this wont work well.
If you are wanting to save the wood a nail puller is in order. Most people nowadays don't even know what one is. The twisted nails used for pallets are difficult to pull with a claw hammer. A nail puller works much better.
Hard to do if your plan is to re-use the wood. They do make a tool called the Pallet Buster which makes the job easier. Costs about $60. Pallets are normally secured with screw nails.