If you're on a small budget, don't buy traffic. Even for a relatively cheap format like Popads, you'll have to shell out a lot of money to find a profitable campaign, especially if you're new to paid traffic. Plus, you have to pay for some tools (spy tools, a good tracker...), you need a way to filter out bot traffic (popads have a lot of it). I made the same mistake as you did when I started doing paid traffic when I had a reasonable budget, so I don't recommend it.