I couldn't agree more. Unless your parts are salvaged from the trash, building won't be cost effective if premade assembled or a kit exists. You can buy cheaply if you get a brand like Mastech. At the very least, you get the hardware you can modify to improve the specs or repair, if necessary.
The reason is economy of scale, inventory and distribution chain. DIY is a 'custom build.' By the time DIY new parts you need reach your distributor, and finally you, its gone through several hands that up the price, not to mention shipping, handing, taxes or duties.
Engineers dedicate careers designing power supplies especially in this Energy Star conscious world. Why reinvent a well established wheel except for the fun of it?
That said when I consider the simple general specs:
linear or switching
one, two or > 3 outputs
programmable
The unit you already have is more than necessary for most home lab work, but do you need more than one ouput? With the ever increasing trend to low voltage, low power usage in devices, especially made to be portable and battery powered, a clear need is one with tight dependable regulation, and constant fully adjustable V and I.
To all those that feel the self-built is the best route to go - can a home built supply even compete on cost with a bought one? There's quite a few high value parts in there:
* Transformer
* Case
* Smoothing caps
* Heatsinks
* Panel meters
I just remember the first audio amp I built from a kit (I think it was 150W or so)- the kit itself cost about £50, built and tested at school using a bench power supply. Then I came to casing it, and found that mains lead, fuse, power switch, grommets, terminals, phones all cost a fair bit. Then there was the power supply... cost an absolute fortune!