Hey Soupman,
Well cheap solutions are most of the times not the best ones.
1. You can get like a pond liner and fill it with water beneed your pots, make sure you lift your pots otherwise they will take up water.
With this option the water will hold bacteria's as well and other bad stuff.
2. Buckets with water but it won't help that much.
The best solution if you want to grow cheap is find the cheapest humidifier, they go for like 20-40euro.
Goodluck growing :)