Total revenue ÷ total marketing spend. ROAS but honest.
MER is ROAS computed across all marketing, not just attributed to specific channels. It includes revenue that wouldn't show up in any channel's ROAS (organic, word of mouth, branded search driven by ads). For holistic marketing performance, MER tells the truth while per-channel ROAS double-counts or misses things.
ROAS by channel often lies (multi-touch attribution is hard). MER is harder to game, it's just total spend versus total revenue. Trend MER over time; it shows whether marketing is actually improving at the business level, not just in one dashboard.