for one or two clutches you can do DIY with just a cooler, and them move everything to a stand up fridge when you have more eggs in a year or two. The big expenditure here is the thermostat--I've got a Vivarium Electronics model, it was about $150, and it held my incubator at a rock solid 89F. Then it's just heat tape and water bottles and Hatchrite and egg tubs, and that's all cheap.

But whether you use a cooler or a fridge, make sure you do put lots of full water bottles in the the bottom with your heat tape... they add heat capacity. Water takes a long time to heat up, and a long time to cool down. Water bottles plus a good thermostat is what keeps you from every having to worry about temperature spikes.