The biggest (hidden) expense is likely your own staff time on understanding and implementing the remediations needed. Pushed hard enough, both companies will give you a price for unlimited scans, which then becomes a fixed cost.
The suggestion that you rollout with targeted groups is sound. Anticipate spending effort in (i) showing them how to use the tool and discuss the priorities and process, and (ii) show that their work has made a difference to justify the funding. Those target groups will also need to invest staff time to understand and fix problems.
In the grand scheme of things, paying a fixed cost to speed up an effort is negligible. It's your staff's bandwidth that will become the bottleneck.
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