Quick Answer: Measure cost per SQL by assigning every sourced or influenced SQL a fully weighted acquisition cost that includes both marketing spend and SDR labor, then apply one attribution model consistently across the funnel. The cleanest enterprise approach is to separate SQLs into marketing-sourced, SDR-sourced, and joint-attributed cohorts, then calculate cost per SQL as total allocated cost divided by verified SQLs in each cohort.
When attribution depends on both marketing and SDR actions, cost per SQL should not be treated as a single blended number without a clear allocation rule. The most defensible method is to define SQL ownership at the moment the lead meets your qualification standard, then attribute costs using a consistent model such as first-touch, last-touch, or a weighted multi-touch framework that includes both campaign spend and SDR time. In practice, you should map all direct marketing costs, SDR compensation and tooling, and any shared overhead into a cost pool, then allocate those costs to SQLs based on source, touch sequence, and conversion influence. This gives you a repeatable measurement system that can reveal whether marketing is generating qualified pipeline efficiently, whether SDR follow-up is improving conversion rates, and what the true marginal cost of one SQL is by channel and cohort.