I use a old deep chest freezer as my incubator. The heating is accomplished by a cluster of 3 60 watt incandescent light bulbs and the controller is a Ranco ETC-111000. I use a small box fan that is cycled with the heater to evenly distribute the heat. I get very steady temps with this system and do not see overshoots that some try to avoid by using PID controllers. The freezer can easily accommodate a deep box where I place the grafting frame. This way there is minimal handling of the cells - I gently pull frame out of the finisher and brush off bees and place in incubator. I make very sure that the temperature probe is placed right next to the cells. I've used this system to incubate 100s of cells without a single failure.
Costs: controller about $50, light bulb fixture (free since I had it laying around), chest freezer (free as my neighbor was throwing it away), but these are often discarded and can be picked up for free pretty much anywhere. The fan, again free since I had it, but these can be purchased on ebay for about $10.