The most ecconomical way is to go with a swamp cooler. Basically a large plastic bucket whatever you're fermenting in can be put in, filled with water. The water will help mitigate temperature fluctuations. To help bring the temperature around fill two liter bottles with water, freeze them, and toss a couple in the bucket of water that your fermenter is sitting in. Replace the frozen bottles as they warm up.
There are many variations of this like using a t-shirt to wick up the water and have evaporation aid in the cooling process. I only used one once and it worked well, but then I got a nice temperature controller as a gift and built a fermentation box out of insulated board and a small frdige along with the temp controller so I can keep my fermentation at whatever temp I'd like it to be. But the swamp cooler is the cheap way. I picked up the big bucket from something like Walmart or Target for under 10 bucks.