An engineers stamp means that the engineer has performed calculations to size all the beams, deck members and connectors and has reviewed or modified the drawings so that the design of the deck is sufficiently clear. It means that the deck meets the building code.
How much this will cost depends on the quality of your drawings. If you are an experienced structural draftsman, it may be a few hundred dollars. If this is the first time you have done structural plans, it will be quite a bit more. The drawings need to meet certain legal minimum requirements, regardless of the complexity of the project.
A structural engineer can give you a good estimate of engineering cost in a few minutes, usually at no cost to you.