It's worth mentioning that it's easy enough to find absurdly cheap (~$20) early-generation dev boards for Zynq FPGAs with embedded ARM cores on Aliexpress, shucked from obsolete Bitcoin miners [1]. Interfaces include SD, Ethernet, 3 banks of GPIO.

[1] https://github.com/xjtuecho/EBAZ4205

Not in the grand scheme of things: you can get fpga dev boards for $50 that are already useable for this type of thing (you can go even lower, but those aren't really useable for "CPU like" operation and are closer to "a whole lot of logic gates in a single chip"). Of course the "industry grade" solutions pack significantly more of a punch, but they can also be had for <$500.

In general, yes. However, the Kria series are amazingly good deals for what you get - a quite powerful Zynq US+ part and a dev board for like $350.

Ages ago I bought TinyFPGA, which is like £40 and I was able to synthesize RISC-V cpu on it. It was fun.