Naor,
I'd advise you to look at this article
http://perkuto.com/blog/marketing-automation/marketing-attribution-model-for-marketing-automation
download this paper on the front page
http://revenginemarketing.com/
Essentially what you want to do is create paired LT/FT source and offer fields:
- FT Original Lead Source
- LT Lead Source
- FT Source Platform
- LT Source Platform
- FT Offer Type
- LT Offer Type
- FT Offer Name
- LT Offer Name
etc...
Then use flows to add a person to a specific Program or SFDC Campaign. I prefer creating Campaign Member Objects, which you can then report ROI on. If you use Marketo's Programs, you'll get detail in RCE, however, that's a lot of Programs. Full Circle CRM can help report on this type of system. I've asked Marketo to consider adding this functionality as well.