The way I think about it whether it is your own business or working for someone else. If the tool can save you at least the money that was spent to acquire the tool, then it is a worthwhile investment. Keeping in mind that saving money can be related to time spent for
- planning (software or otherwise)
- design / development
- keeping records (eg. time to track history of bug / source / business admin)
- monotonous tasks that a tool can perform (backups?)
- concerning yourself with problems that a tool can do for you.
Don't forget that a tool generally requires initial training / learning that is a cost also. So if the tool is a once off, you may need to weigh up whether the cost of learning outweighs the cost of the tool.
Getting back to the basis for your question... As an example, if a build tool was $1000, your time is $100/hr. We will disregard tool training cost as we intend to use the build tool multiple times.
If you need to spend 0.5 hours with a tool to create a final build environment the maths says
Your cost with a tool = $1000 + (0.5 x $100) = $1050
If you need to spend say 12 hours to do set up the build environment manually
Your cost without a tool = 12 x $100 = $1200
Or, maybe an example that is a little more realistic, you can see 6 upcoming projects. Each project will take 0.5 hours to set up with a build tool, or 3 hours without.
Your cost with tool = $1000 + (6 x (0.5 x 100)) = $1300
Your cost without tool = 6 x (3 x 100) = $1800
It seems that the build tool in these scenarios would be seen as a worthwhile investment.
Hope this helps...