Also..

" it ends up writing or customizing quite a bit of code.

That's not cheap."

Should also mention if costs of (a) writing completely custom software or (b) licensing software from ibm/microsoft/whatever compared to said customisation of existing open source code.

Having worked in such a startup at it's growth stage I can say that licensing at scale is much much more expensive than customisation and setup of existing open source code.

In fact, this company scaled at a point where it needed to either expand its oracle license or custom built code that would take load off oracle DBs so the license would not need expanding. They custom built the code using standard tools and open source technology because it was cheaper.

Scale works both ways. Licensing in my personal experience turns out more expensive.