Measure the effect of retention events in four ways:
Churn rate comparison. Event attendees vs. non-attendees. Upsell/cross-sell. Do participants take on extra services more often? Net Promoter Score. How likely are they to recommend you? Engagement. Do they open newsletters more often and respond faster?
Build a simple dashboard per client. After 2-3 events you have enough data for ROI. More in our article on client retention through events.