It's for an open source project, not an internal work instance or anything. As a student at uni with no full time job, adding more monthly charges is hard to deal with.
At Georgia Tech we got a very small server space which was used in a few CS classes, it would have been large enough to run hubot. Your university might have a similar setup.
Heroku was basically becoming the go-to place to run thousands of little bots—and more importantly, to advise people to run their little bots—without thinking about marginal costs. It was becoming everyone's little pocket of persistent personal cloud agents. The "launch on Heroku" button on Github pages was a thing you could just press on a whim as a realistic solution to your problem.
IBM's Bluemix has a very generous free tier. If you can run an app or apps using a total of 512M of ram with modest data storage requirements, then that is free.
Old computer, yeah- Raspberry Pi, not so much. Takes hours to compile node from source, and I've only been able to find one source for ARM node binaries.
And after all that, hubot still ran pretty damn slow. :(
Still, Debian only ships nodejs 0.10 (even Sid) for now -- and I'm not sure what the status is for compiling node 0.12 on armv7 (it apparently has issues on armv6 which the rpi v1 runs).
Fwiw, if you can get by with node 0.10 for hubot (Which I think you can[1]?) -- there shouldn't be any problem:
