Unfortunately often the problem is the opposite. You may spend 10 units of time integrating a 3rd party system into your workflow versus 100 units of time writing your own custom system, but then you'll spend 1000 units of time uncovering gotchas in the 3rd party system, fixing them - which means learning system outside of intended scope and developing your own code, or inventing workarounds for less than perfect 3rd party system, or skipping some functionality in your workflow because you need something which you can't get from 3rd party tools etc...
Not core to your business? Doesn't mean you can't waste both time and money buying.
In MS Excel.
How about an option C, shop around? Profitwell for example has reasonable SaaS metrics and it cost $0. Seems like the best of both worlds, no code for me to write and $250 in my pocket.
If you’re technical yourself, build it for no cost and save the $7,500. If you’re deciding between buy/build, it’s a wash on this imaginary scenario so do whatever you like better.
Point is, it all depends.
My advice: If the tool is right, buy it. If nothing out there fits your wants, then build.
There's a big advantage to controlling your own stack. It's why companies like Google built things like file systems.