To what extent is implementation a consultant's concern?

Hi all,

Something I'm struggling with a little at the moment, and would appreciate advice and experience.

I'm trying to design a solution for a client, and in order to do so am interviewing various employees to gain information. During this process, it becomes clear that there are challenges related to the implementation of my proposed solution ("It's too difficult/time-consuming").

How do you strike the balance between recommending what you think is in theory the best solution, and what you think has the most realistic chance of being implemented? And aside from that, how do you 'factor in' costs of implementation into your solution? (Costs would be resource-related but not directly financial)

Thanks in advance. Interested to hear your responses.