About the easiest and cheapest thing to do is just get a cooler (6-20 bucks depending on the size), thermostat (30+), and some heat tape (clips, wire, aluminum tape and heat tape should be like 15-30 bucks) for the incubator. Toss a couple bottles of water in to hold heat, then a plastic tub, substrate, and press-and-seal. It's pretty common as a setup and works well.

If your female's husbandry is up to par and you don't mind her not eating for an extra 2 months, you can always maternally incubate. It essentially costs a negative amount as you most likely will save on 1+ rats, and a lot of people who do it have good results. Mind that your husbandry needs to be solid to pull it off, but she can control temperature and humidity (increases to both, she can't really decrease it).

The only problem I had with maternal incubation was cleaning related (it can be a pain to clean around her or move her and the eggs to clean)