I would put it in terms of traffic price from the keywords you are targeting. You can go back several years and itemize the KWs that have driven conversions and back into $$ by rank tracking for those kws and the assigning a CPC value based on position (there is CTR % based on position). That can give you a Traffic Price (similar to SEM Rush: http://screencast.com/t/5qatp46msWsJ
For the with / without seo question, this is a non-starter. There is no way to gauge this because there is no way to tell what will happen in the next year. This has more to do with risk to me than comparison. If I have cancer and I go to my family doctor vs. a specialist, how can I quantify whether I will live or not. I feel less risk in selecting the specialist since I do not speak that language.
This is a teachable moment for your client.