My current project used 4 x DS18B20's

http://www.picbasic.co.uk/forum/showthread.php?t=12712

All you need in addition to the DS18B20 itself is a 4.7K resistor and obviously some means of displaying the temperature it reads - either an LCD display or serial to PC and use Hyperterminal. If you chose the LCD screen, then estimated cost would be less than £25 for the sensor, resistor,LCD, case and (depending on the PIC) the PIC too.