Earlier quoted context omitted.
I don't read this as when open-source was invented, but when it happened for the corporate world. In 2002 it was a very reasonable choice for $BIG_COMPANY to use a proprietary web server, e.g. IIS. In 2008 that would have been really be weird.
But why did that make development cheaper? An enterprise copy of Windows with IIS cost maybe a thousand bucks, right? Maybe there were more costs, my knowledge is, y'know, 23 years out of date.
Now you can start developing on it…
With open source it’s not just the cost of software you save, but also potentially all the other bureaucracy that you save due to not having to pay money to do something. You also get a lot of transparency on the technical side about the products you may choose to use.