probably depends - I tend to build my own servers (it's a bit like building a desktop computer - I don't really need an enterprise one) but a used dell or IBM server isn't that expensive - they probably use a fair bit of power and they are not quiet but probably cheaper than what I build. (I just go with an xeon E5 12xx cpu and ram/ssd/powersuply and a 2U chassie for the rack - and I tend to buy supermicro motherboards) but if you want something low power that should make you able to play - go for an intel NUC with an i5 or i7 loads of ram and a ssd and maybe an m2 ssd
:) I've got a Pie that I'm planning to use on a ups so that it gets network attatched :) they are fun to play with - and not as expensive as a fullblown server
I've got a dedicated webserver under my desk for these things :) makes it a lot faster - I used to have it on an atom based computer but that was a bit of a challenge (it surviced a thungerstorm minus an ethernet port - so had to run it with a usb ethernet adapter - not ideal) - not I've got it running on a beefy computer with zfs