Programs must have period costs otherwise they won’t be included in RCE reporting unless you set the analytics behavior for a channel to inclusive. However, if you want to filter by cost time frame or if you want to see ROI information in the RCE Program Opportunity Analysis report, each of your programs must have period costs.
I recommend adding multiple months at the same time when you set up a program for ongoing campaigns like web content. The cost should only include the cost of hosting an event, developing the content or promoting the content.
If you want to compare in Marketo how two or more advertising campaigns compare against each other, consider creating one Master program and multiple shell programs each with their own cost.
Master Program contains the form on a landing page, email assets and smart campaigns. Don't set membership or success in this program. Use URL parameters to set membership and success in the shell programs.
Shell Program A - Facebook advertising cost
Shell Program B - Linkedin advertising cost
Shell Program C - Corporate web site with no cost
A great article http://nation.marketo.com/blogs/marketowhisperer/2016/05/08/why-you-need-period-costs-for-reporting on the importance of period costs.
Link to Checklist for Attribution Reporting.