render farm newbie questions
I'm not an economist, but I have built and run a small render farm. I'd make the decision on a ROI basis: will you get back a return from your investment? I can't answer this for you, you'll need to add up all the costs for both options and compare. 

Costs for a physical render farm are mostly fixed - i.e. the capital outlay, but you have to factor in depreciation, maintenance and electricity, and the price of housing it. Costs for a cloud-based solution are all on a usage basis.

So a cloud-based solution would work if you only need it occasionally, especially if you need massive render power that would cost a lot to set up and run. Commercial render farms can have thousands of processors, meaning that rendering a sequence takes only as long as it takes to render the longest frame. So cloud farms have the edge in terms of scale. A con with cloud based systems is that you have to trust a third party with your intellectual property. There's also the overhead of sending and receiving the files, depending on your internet connection (a bit of an issue here in Australia).

A physical render farm makes sense if you're using it all the time, and you can afford the upfront cost for the size of farm you need. It has the advantage that it's always available, and using it is (almost) free, so you can use it for things you wouldn't bother sending to a commercial render farm. After all, the more you use it, the more your return.

An inexpensive way of building up a render farm is to retire workstation machines to the farm, thus adding extra value to them. The downside of using older machines is the energy cost - generally they do less computing per watt-hour than newer machines.