I've purchased web apps and websites (and have colleagues who have done the same) for $1k-$2k that today make right around that each month. But those are the best results; that's not the most common outcome of purchasing in this range.
The most common purchase price is between $2k and $7k. Though I do feel like that's been creeping up over the past couple of years as this approach of buying becomes better known.
The more you can spend the better off you are; everyone's trying to buy in the $2k-5k range so there is far less competition in the $10k-$25k range. But $5k-$10k can also net you a nice app.
In addition, although this approach saves months of dev time it does take time to find deals. You either have to sift through sites on Flippa, which I did for years, or find deals and cold email like I did. It's not something where one day a deal just drops itself into your lap with no work invested.
The hardest one is the first one - once that's going you can use revenue from that to buy the second, and so on. This most recent acquisition was funded purely with revenue from the other 8-10 apps I've bought and revamped over the past several years.