Building a temperature-controlled water bath
You can build this with a very simple op-amp comparator circuit:



This uses a thermistor as the temperature sensor, and a potentiometer as the thermostat setting. You then just need to hook up your heater to the relay's switch contacts.

You'll just need to calibrate your pot by testing the water temp with a thermometer in different positions of the pot.

The parts should cost no more than around &euro;11/&pound;10/$18. You can power it with an AC adaptor (might be a good idea to add a 470uF capacitor across the supply as well. You can then use the rest of your budget for the heater.