Some of the best-run ventures in the world use custom software to operate more powerfully and efficiently:
Of course, you likely know some or all of this already. You may even be using general-purpose software to do some of it. What you might not have, though, is the advantage of using software created specifically for you and what you want to do.
When you use software that wasn't created just for you, what eventually happens is that you either ignore a bunch of elements you don't need, or have to live with a limitation that requires awkward external workarounds. A good example might be something like a monthly report. You can build it in a general-purpose spreadsheet, but that means getting the data from somewhere, pasting it in, and wrangling your own formatting. With custom software, an application is created where you press a button labeled "Generate Report," and the computer does everything for you.
This is where I come into the picture. I've worked on teams (BlueDoor Software and Pando Development) building enterprise-class internal and public-facing applications for mortgage and manufacturing companies. The solutions we created were tailored specifically to their needs, not cookie-cutter setups that we inserted some graphical changes into. I've also created my own toolkits for constructing applications. In fact, this website is an application I built for myself, running on an API framework that I architected essentially from the ground up.
The only honest answer to this question is "it's situationally dependent." A relatively simple website with limited interactivity might come in at somewhere around 30 - 50 hours of development time and $1200 - $2000. A complicated application involving business logic and a database can take much longer and cost much more. Custom software is an investment, and often a very significant one. The payoff can be very high, and that has to be balanced with what you can afford.