Online vs. Print Marketing

My department just held its annual fundraiser last week in which we had over 150 people attend. It's a big deal for us given its revenue generation, and is probably our most widely marketed event all year. I'm now in the process of recapping the event, and making notes of things we should change for next year while it's still fresh in my mind.

One of my concerns is whether the ROI is there for our hard copy invitations we mail out. As you can imagine, the costs involved with designing a print piece, printing, and postage are anything but cheap. We also heavily marketed the fundraiser online via email blasts, our blog, social media outlets, etc.

When all is said and done, we are only aware of 2 registrations that we received from the mailed invitation (attendees are asked to answer how they heard about the event at the time of registration). My first reaction is that the mailer wasn't the best investment/use of marketing dollars, but I have to remind myself of a few things:

1. While most people selected that they heard about the event online, they may have received an invitation which reinforced the original message, or reminded them to register.
2. Until I get a chance to compare our registration list to our mailing database, it's hard to justify my original assumption.
3. If it turns out that our registrations were mostly from people who would not have received the mailed invitation at all, then it may be wise to reconsider doing a mailing for next year.
   

My main quesiton moving forward, however, is if most registrants did in fact receive the mailer but listed "online" as the source, how can we justify (one way or the other) spending money on that campaign?