Any heat which is transfered through contact is easily the cheapest. As meantioned by Wokka, heating the environment results in a lot of wasted heat. The best option is to either use heat cord or heat mat. If you get inventive you may be able to use one cord to heat multiple enclosures. In addition to the pvc pipe suggested by Snowman, there are also heat mats that can be made from CoreFlute and heat cords. I think Snowman may have made a post about it a few years ago. Microchips used for the pet industry (i.e. Cats and Dogs) are usually >2mm x 12mm. There are also Microchips that are 1.3mm x 8mm which are used for wildlife monitoring etc. These may have been the ones Lawra was refering too.