I don't know if they're the cheapest but I buy most of my paints from Miniature Market. Vallejo paints are $2.89 and shipping is usually $6 if your order is less than $99 (otherwise it's free). They have a pretty wide selection so you can pick and choose whatever you'd like. Reaper and Army Painter are a little cheaper per pot. Going this route might end up slightly (like a few dollars) more expensive than buying a bundle with Prime shipping, but you'll only get paints that you actually wanted. IF you're like me and end up buying a bunch, keep a spreadsheet to track which colors you already have. I've ended up with a few duplicates before I started tracking things better. Plus, you can pick up some inks, which if you follow someone like Sorastro (who will eventually be doing a Kingdom Death series), play a big role.

I'd also recommend you get a can of Army Painter Quickshade. The can isn't too big but it will last a long, long, time.