That question decides what gets built. ResourceEstimator.com is the general home for answering it: quantum workloads today, classical and cloud workload costing as the engine generalizes. Part of the QubitIaC family.
Interactive fault-tolerant cost calculators for Shor, Grover, and quantum chemistry, grounded in the published literature (Gidney & Ekera, Fowler, Reiher, Litinski). quantumresourceestimator.com
Validate and visualize Quantum Resource Estimation Format programs: QREF Render and the QREF Guide.
The same discipline for classical workloads: what a training run, a migration, or a scale-up actually costs before you commit. In development - this page will say so plainly when it ships.