To maximize earnings on Perk or Swagbucks, you must have at least one phone you can leave alone, just running videos, or one that you can dedicate to searching or doing tasks (of course it can do double duty with Checkpoints or any number of the other income apps).
Ideally you have up to five phones (the max allowed by Perk and Swagbucks), or what’s known as a “farm”. Keep in mind Swagbucks and Perk require Android 4.0+. If you don’t have five devices lying around, cheap Android phones are the best bet to keep your earnings high.
The first place to look is C7 Recycling. They even have a list for “income apps”. After that, I’d check Best Buy, Kroger’s, or Walmart. They often have sales to cull inventory, where you can pick up a decent phone for $5-$10.
Keep in mind, while you don’t need the most powerful phone on the market, a good amount of RAM is necessary to keep the phone from hanging – the idea is “set it and forget it”. A phone might cost $5, but if you have to constantly babysit it, you’re not going to make the money you want out of it with ease. I wouldn’t buy a $50 phone for Swagbucks or Perk, but I would buy a $10-15 one.
At current rates, a $10 phone will break even in around two weeks or less. Right now, the oldest phone I have in a farm is an iPhone 4s which has run Swagbucks and Perk videos every day for about five years. The oldest Android I have is a Kyocera Event (I would NOT recommend this phone unless you want to spend time tweaking it, I had to do a lot of tinkering to get it working the way I wanted to) which is about three years old.
All of my phones have been used when I bought them, and all have more than hit their ROI mark, and earn more than the power they use to run.