The cost of your digital strategy usually lines up with the cost of your website or app. If you start with something small, then your digital strategy will be on the less-expensive end of the spectrum.
Costs can increase as the size of the project grows - but so does the return. For the most part, digital strategy costs as much as you can - or want - to invest in it. You can always begin with a smaller project with a low budget and expand from there.