Certainly for an agency to do this will cost them more than an hour of developer time. $120 per year is a steal, OP should add different pricing brackets to capture more value from bigger agencies.
I 100% agree this would take more than an hour to develop with a nice UI, and there's some ongoing maintenance, but it's still pretty low numbers. And there's still arguments for a DIY approach - for instance, Aa DIY approach means you can improve/iterate/integrate at your own pace.
Each hour you spend improving/interating/integrating your DIY app is an hour of developer time that you can't bill for.
You'd easily spend over an hour per year on a DIY app. Agencies charge at least $120 per hour so the choice between building yourself or paying someone else to do it is a no-brainer.
At the enterprise level $500 is the amount employees can generally spend themselves without getting approval, so there should really be a plan at that level of pricing too.