The 1-to-4 multiplier
One environment in production is a one-unit bill. Four environments (dev, qa, stage, prod) is closer to a four-unit bill on per-environment platforms. The exception is consumption-based tiers, where the per-environment overhead is small.
Collapsing non-prod safely
You can usually collapse dev + qa into one environment with namespacing without losing isolation. Stage and prod should stay separate. The savings on three platforms with per-environment pricing can be 25% or more of the gateway line.