[SOLVED]Water pump control based on temperature

Someone asked me to make something that'd turn on/off their water pump based on water's temperature. The pump is for circulation around the house, water is heated with heater. When water gets colder than x, it should turn off the pump.
So I decided to use arduino for this. What is the cheapest way to do this?

The cheapest will be probably buing soome arduino (uno for example), some thermistor and one relay and one transistor (for controlling relay) and sme resistors. That should be everything and if you buy this items on ebay so it will cost about 25$.

and a diode for over the relay!

check - http://openenergymonitor.org/emon/buildingblocks/mains-ac-relay-module -

use 2 diff temperatures, one higher to switch on and a lower to switch off, to prevent a relay vibrating

void loop()
   int t = temperature(); // thermistor function

   if (t >=25) digitalWrite(RELAYPIN, HIGH);
   if (t <=23) digitalWrite(RELAYPIN, LOW);