Here is a short answer: Digital strategy can be created internally or externally (with the help of outside partners). Internally, depending on how big your organization is and how transformative your strategy is, the costs could vary greatly.The cost of the internal strategy is determined by: the cost of the time of the people that are involved + any opportunity costs that are incurred by their use of time on the digital strategy project vs. their ordinary duties.
Externally, the costs are far more measurable, but again will depend on the size and scope of the objective. At Atlantic BT, we make it easy by setting up a low-cost collaborative whiteboard discovery. The purpose here is to clearly define the objectives and scope of the digital strategy and come up with a few ideas for things the team would like to see incorporated. From there, everyone gets excited about the overall strategy initiative and we are able to scope the costs effectively.