For most businesses that deal with 1:1 selling through a sales force (B2B and large ticket purchases), marketing activity and sales activity can be connected via lead records in CRM. In the case of consumer mass marketing (i.e. retail or dealer sales), statistical inference is typically used to determine the correlation between marketing activity and sales activity.
Organizations and business models vary widely, so our ability to determine your ROI is dependent on the quality and availability of your sales and marketing data.