Hi Justin,
Some potential solutions for you (assuming you have Salesforce.com):
1. You should be able to get this number by building a Smart List in Marketo. I would use these filters:
Black Listed = False
Unsubscribed = False
Marketing Suspended = False
Email Invalid = False
2. If you are using Salesforce and syncing your Programs to Campaigns, you could have an Apex trigger writen to calculate the number of Campaign Interactions in a field on Lead / Contact. Then you could run a report to get the average.
3. Make sure that you have enabled and configured your Campaign Influence rules, this way Campaigns are getting associated to Opportunities. Then you could run a report to show you Closed Won Opps Without any Campaign Influence records, compare that to the total Closed Won Opps number to get your percentage.
4. You would need another Apex Trigger on the Opportunity to calculate the number of Campaign Influence records and put that into a field on the Opportunity. Then run a report on all Closed Won Opps, and get an average of this field. Compare this with the Average Campaign Interactions of Leads/Contacts not associated to Opportunities.