I have a raspberry pi with 5 ds18b20 temperature sensors in the attic. I originally put it all together to see the difference a couple fans made, then I forgot about it, then I didn't feel like going up and pulling it out of the rafters.
It was less than $50, and with a small amount of googling and learning, in less than a day I had it running well enough to pull a list of temps and times out, and see what I wanted to see.
If you spend a bit more time, or are smarter than me, you can have it send alerts or show you a graph or light up a bat signal.
I couldn't find anything on the shelf for less than hundreds of dollars to do what I wanted. 3 years ago.