I am using a thermoelectric cooler from a pc's heatsink to produce electricity. Its size is 30mm by 30mm. I will cool it on one side at -10 degree Celsius and the other at 24 degree Celsius. Can anyone tell me how much electricity in watts will be produced. I'm using this heatsink cooler because it's the most easily available to me. Can anyone suggest an even better thermoelectric generator? Also how much electricity it would produce and how much it will cost?
The details of how your particular device will perform is very much a function of how that device was constructed and how it will be used, but in general, the physical effect is called the Seebeck effect -- briefly, the conversion of a temperature difference across a device to a voltage.
Efficiency for such devices is actually quite low, with $\eta = 0.02$ (2% efficiency) being a typical value.
See this manufacturer's web site for many more details. You may also wish to search for datasheets for particular devices (they're usually called Peltier coolers after the inverse physical effect and more typical application) to get more details about specific device performance.
You'd be lucky to even generate 1 Watt. Compare yours to this much larger 10 Watt generator intended to operate off hundreds of degrees of temperature difference: