And that is also what many low-budget developers get burned on.
Devices that cheap tend to be that way because their OS support has dropped off a cliff or will soon be. With Apple devices, this kind of drop is often a workable compromise for civilians, but a deal-breaker for developers. You will soon find yourself unable to keep your products on the shelves because the store demands a recent enough version of XCode, which also requires a recent enough version of Mac OS.
For iOS and Mac development, the most affordable option is a Mac Mini. Since you recommended buying second-hand/refurbished, let's shop around and see how well your advice holds up. Here are my first 3 Google results for "mac mini refurbished":
The price point of $500 that you recommended (I assume you missed a 0) is where I take the most issue with. See how those "deals" offer 2012 and even 2010 models? Go for those, and you'll be screwed in a month or two's time. Big Sur, the upcoming version of Mac OS, requires at least a 2014 model.
Mobile devices are even more prone to this kind of issue than desktop devices. I made a similar mistake myself in 2016 by bidding for an iPod Touch (4th Generation), which GMS 1 stopped supporting just about a week after I received it. XCode 8, which was then the latest version, cut off the iOS version and no longer compiled to it. In 2018 I made the mistake again by purchasing an iPhone 6 on discount. iOS 13 cut the cord on it less than a year after I made the purchase. The $250 that has gone into both of these write-offs could have gone toward a device lasting much longer in battle.
So please, before you go for "deals" like those, always be mindful of official OS support drop-dead dates. You can easily waste more money than you save by churning second-rate devices that expire too soon for development use.