You can get the basic wall thermometers for $2 from dollar shops etc, they are perfect for using inside an enclosure (mounted next to the thermostat). I also use the twin probe digital thermometers available from Herp Shop/Animal Attraction for monitoring temps inside tubs etc
There are some nifty little thermometers which have a suction sup to stick on the wall or glass of an enclosure at Target (or was it Kmart?) which sell for about $3. Obviously they are not going to able to measure the tempterature in the middle of the enclosure (just the side) but it is handy for getting an idea of the temperature. I use a thermostat with a probe for the heating though.