Honestly, I don't... and I'm not sure I want to know.
I based my build almost completely
johnodon's old build, using the
schematic so generously provided by
P-J.
There are already some prebuilt and "commercially available" control panels.
eBrew Supply has a kit, and
High gravity Brew has a pre-assembled CP, for example. I decided to build my own, sourcing the parts myself so that if it broke, I'd be familiar with the insides and could better troubleshoot it... that, and I could spread the cost out over a few months and really customize it so it made sense to me...
I'd budget between $1000 - $1500 for it... that would be for everything.. stand, kettle, element, wiring (the 30a GFCI and lots of 10/3 wire), control panel, the 15-gallon plastic conicals, chiller.. but if you can spread the build out over a year, that's $100 a month.
I do think the key is to put room in the budget each month for the build, and, like so many of us, spread the cost out over the course of a few months.