Event ROI is the return an event produces measured against what it cost, calculated from pipeline, revenue or whichever outcome the organizer committed to.
Event ROI is only calculable against something you committed to in advance. Pipeline generated, deals influenced, renewals retained, members recruited, credits delivered. Choosing the metric after the event means choosing the metric that makes the event look good, which is why so many event ROI numbers are unfalsifiable.
Venue, catering, AV and platform are the obvious lines. The ones that get left out are staff time, travel for the team rather than the attendees, content production, and the opportunity cost of what those people were not doing for six weeks. A cost base that only counts invoices flatters every event.
A deal that closes four months after a conference touched a dozen other things. Single-touch attribution to the event overstates it, and ignoring the event entirely understates it. Most teams land on influenced pipeline, reported as influenced rather than sourced, and are explicit about which they are showing.
Knowing who actually attended rather than who registered, which sessions they sat in, and whether that data reached the CRM. Without that the ROI conversation is an argument about anecdotes. See event analytics and reporting.