If you wanna break it down by "systems" the most expensive part is the regulatory compliance you need to engage in before you start.

People shit on the healthcare industry for being written into law and they shit on the credit processors for taking a cut of every transaction but the degree to which various flavors of paper pushing engineers are required by law to be given a cut of every single creation, alteration and major repair of a structure is mind boggling.

And this racket is a large part of what puts the squeeze on low end projects.

If it takes me $50k of engineering to be granted a $100 permit to do another $50k of tree clearing and dirt work on a lot then whatever I build needs to make up for that. If not for BS compliance those numbers would be something like $5k, $100, $20k

All of this expense, levied upon literally every single project everywhere, all to prevent the 1/100 or 1/1000 case where someone does something (that they likely knew was) stupid and causes a runoff problem or whatever, that could likely be mitigated by the same costs after the fact where and when it happens.

And this is before you start discussing all the areas where industry and company lobbyists have got their clients inserted into the IBC at the expense of the public.