Does anyone know how to make a Sous Vide style cooking vessel?
You could probably do it on the cheap, but the temperature precision might suffer in the process:


($0-$3) A container for water that can handle being warmed up a bit. (recycle something or buy a 5 gallon bucket)
(~$20) A small submersible garden pump.  (I have no idea what temperature they're rated for, but I use one in my greenhouse hydroponics systems, and it gets rather warm in there in the summer)
($10-20) An immersion heater
(~$65) : a thermostat control unit


Total : ~$100

If you're comfortable wiring up your own circuits, you could likely make your own temperature control unit for less, as that's the bulk of the cost.

If you're not comfortable with all of this, then the Anova Precision Cooker is under $200. (currently $25 off, bringing it to ~$155).  You'll have to supply your own water container.

Update :  Ecnerwal got me thinking -- vessels with thermostat controls.  (heater is easy, it's the thermostat that's expensive).  Crock pots have the unfortunate problem that they only have one to three present temperatures (low, high and warm).  There are, however, a few things that might be useful :


a deep-fat fryer.  Preferably cleaned, or one that can be disassembled to clean.  They tend to have very powerful heating elements, so you may need a screen to ensure that the pump and the food don't get to close to it.  You'll also want to check the dial for the lowest temperature that it can maintain.  (it might not go below 200°F)
electric skillet.  They're quite shallow, but if you're just cooking a steak for 1 or 2, it might work.  (again, check the dial to see what the minimum temp it'll hold is).  Depending on he size, it might be possible to put another container inside it to hold more water.  
Coffee urns.  They can be much larger, but they typically just have one setting, so you'dd need a termostat on it.