Right. Fpgas shine if you have high data rates (multi Gigabit on single/dual pins), low latency, lots of similar computation (multiply accumulate), a lot of complex pipelined logic and if you need to be really compact at the same time.

The effort to get this running is also crazy huge. Large designs "compile" for hours, simulation and in situ testing can take months. If your fpga has 1000 pins, even PCB design, manufacturing and debugging is hard, if you even have a sufficient scope at hand. I would guess 10x compared to solving a problem with an embedded cpu, which is 10x compared to solving a problem on a standard PC. Expect another 10x if you go from Fpga to custom ASICs.

I'm curious what could be expected / possible with 5-10k. Tweaking an existing design, on proven and available hardware? Testbench for a module only? Maybe "just" VHDL/verilog coding (instead of dealing with the Fpga)?

(sorry, no marketplace)

Many of the common usecases, like camera input, will have dedicated peripheral a available in a System on Chip or microcontroller. Which would then generally be preferred over FPGA because it will require less custom development, quality assurance, etc.