On the cheap I'd suggest running any old multicore desktop with a big usb hub that you can present as various drives, with as much cheap ram as you can cram into it, so you can practice setting up a domain with several machines.
Run everything on a VM so you can blow it up without consequence.
Server 2012 evaluation edition which is good for 180 days.
SQL Server 2012 developer edition which should be under $100.
Agree with Benjamin, here...though I have a DELL R910 and a shed-load of RAM and Storage :-P ...and being able to create a virtual network domain and play around with VMs of all types, DCs, SQL, SSRS, SSAS, Sharepoint, IIS is fantastic, as I can create snapshots, play around - and possibly screw things up in the process - and then restore back to the snapshot to do it all again. :-D
...got caught with this being an 'old' thread. My excuse is that I've never ventured into this part of the forum before, the parts I usually view are much more active, and from what I'm seeing I don't think many others do either.