Here's a webage showing how to build a fridge for meat curing. It should work for dry ageing.

I made one last year for dried sausages and the only modification I made was to put the humidifier outside the fridge, (I put it on top) and I fitted a 1/2 inch tube to bring the moisture in through a hole I made on top of the fridge. I also made two holes in the front door to put a small computer fan and an adjustable trap for air circulation (the fan is on a timer and runs 5 minutes twice a day to change the air).

I found it difficult to get a humidifier and temperature cotroller under 100$ in canada, there fairly cheap in the US though. I got mine from this store on ebay:
So far there working great. However, you do have to do some electrical wiring, they're not "plug and play", but it was easy even with basically no electrical knowledge.