Total revenue divided by total marketing spend; a top-down, account-wide blended measure that sidesteps the double-counting in channel-level ROAS.
MER, the marketing efficiency ratio, is total revenue divided by total marketing spend. Northbeam defines it as a top-down, account-wide blended measure, unlike campaign-level ROAS (northbeam.io). Because it uses your real top-line revenue and your real total spend, it cannot double-count the way summed platform ROAS does. Triple Whale publishes the closely related blended ROAS as total order revenue over total ad spend (triplewhale.com).
MER answers a different question from ROAS: not 'did this campaign work' but 'is the whole marketing engine efficient'. Keen Decision Systems gives a rough benchmark of a good MER at 3.0 or higher, meaning about A$3 of revenue per A$1 of total marketing spend (keends.com). Treat that as a starting reference, not a target; your real threshold is a break-even MER derived from your blended contribution margin.
Worked example (Demonstrative): A$100,000 total revenue over a period on A$25,000 total marketing spend gives MER = 4.0. If your break-even MER (1 divided by blended CM%) is 3.0, the period is profitable on a blended basis. If blended CM% is weaker and break-even MER is 4.5, the same 4.0 MER is losing money.
Blufire converts MER from a vanity revenue ratio into a profit gate by computing the break-even MER from your blended contribution margin, so the account-wide number is judged against the margin it produces, not against a generic 3.0x rule of thumb.