This is slightly left of field, but one thing I know a lot of businesses don't consider when it comes to ROI reporting and program cost is the internal resource cost of a program, which I think is a real missed opportunity - looking at just the hard cost can give an incomplete picture.
As a completely arbitrary example, looking at hard cost and revenue, you may see that a content download had 2x the ROI of a discount promo email send. But you might find that the internal resource cost of the former is much higher than the latter, and that you could create 4x as many promo sends for the same resource, resulting in double the ROI of the apparent winner.
It's a perspective that easily gets lost once the reporting gets a few steps away from the people who manage Marketo, and how you manage it really comes down to your business (do you give each program type/channel a standard average internal cost? Do you approximate hours on a program by program basis? Have a standard "cost" per hour?)
But other than that, my real things are - make sure you keep channels and program steps simple and flexible; ROI reporting is important at so many levels of an organisation, it's really important to be able to get board-relevant high level and user-relevant granular.
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