How to structure an exploration offer?

Dear fellows, 

For the past 6 years I have been running a digital marketing agency, but now I'm moving into consulting. I find that my specialized skills add a lot more value to my clients this way and the trials have been going well.

I read somewhere that it's good to create a discovery offer, which also makes sense from a new business development perspective. For my exploration offer I want to analyze their current marketing infrastructure, and I'm sure I could always find some small improvements that could make an immediate impact. After that, I will also have a good idea of would it would take to take their results to the next level long-term.

Now my main challenge is that I have no idea how to structure the analysis. Both in delivery as in the proposal. I would like it to be something fixed and standard for a few thousand dollar that I can easily sell to new potential customers. Does anyone have an example, an informational resource, or perhaps some ideas?