When I'm adding "Period Cost" to a Marketo program, it ends up in the "Actual Cost in Campaign" in SFDC.
Is there a way to update that info into the "Budgeted Cost in Campaign"? as I'm updating retainers of a 3rd party for the whole year and I want it to gradually appear as the year goes in the Actual cost.
I'm afraid it isn't, no. Only the Actual Cost field is synced to the Period Cost specified in Marketo.
Not from Marketo. You could run a process builder in Salesforce that updates Budgeted Cost if Marketo puts a value in Actual Cost.