> I have a fully staffed team it’s not 5m a year. For a company twice his size.

I don't think the numbers OP chose really matter. The point is to do the exercise. Try and work out some numbers on how much the current software is costing, how much room there is for improvement and how much investment it will take to get there. Add a big margin to account for all the uncertainty with building something out.

If the numbers still work out significantly in favour of writing your own software then it may be a project worth considering.

What does on-call look like for you?

Most business do not require on call for their internal tools

Depending on the area of logistics OP is in, it's not unlikely that he'd need on-call engineers for his logistics management system. Blockages in supply chain can be extremely expensive.

At my company, downtime is approximately $50k/hr in a 24/7 environment for manufacturing and logistics. We're ALL on call.

It was brutal, but the mentality was - it's brutal now but you have the power to fix it, so engineering hours went into fixing broken windows.

That adopted mentality pays off tech debt fast.

Sure; but eventually you've got a bus number problem and a "I want to be able to take a vacation and I want my coworkers to be able to take a vacation" problem, and that requires more people.

At least that's how I feel about team sizes.