What's preventing the FPGA AIB from becoming a commodity like a GPU? Is it just economics of scale, or are these things fundamentally more expensive?
Supporting PCIe and other high-speed interfaces (10Gb ethernet and beyond, etc) requires physical transceivers which look a whole lot more like "wireless communications over a PCB trace" and less like the traditional "drive a digital signal over a GPIO pin". These interfaces also typically drive requirements for data buffering as well as higher clock rates: all of these things increase size and cost.
That said, you can get basic chips that will do this for much less than "thousands of dollars" - Xilinx's Artix line is optimized for low cost + relatively high numbers of transceivers compared to the number of logic cells. You'd probably be interested in a development board something like the PicoEVB, which is in the USD $200 range and provides a M.2 form factor / PCIe interface to a FPGA. The actual FPGA itself to do this can be had for less than this... but the cost of the PCB, connectors, adding DDR memory, etc., etc., do start to add up.
I think there are a few fundamental problems with FPGA compute offload. First is that, as niche products, FPGAs are always a node or two behind the leading edge, so their logic gates run slower than CPUs and GPUs; even their "hard" compute blocks are not as fast. Second, the fundamental nature of FPGAs as a "sea of logic" mean that routing delays reduce your max frequency, or make pipelining necessary thereby incurring latency. Third is memory; historically, FPGAs have not supported high-bandwidth GDDRx, and if you're crunching on something you generally want bandwidth. The latest high-end FPGAs do have HBM but they are quite expensive.
So why would anyone want something that's slower and more expensive? Well, you have to be doing something special, like a lot of custom parallel processing pipelines, or with hard realtime requirements. Basically, a niche, and that doesn't lend itself to economies of scale.
https://www.element14.com/community/community/project14/hard... links to more or less everything you need to know.
https://store.tinyfpga.com/products/tinyfpga-a1
Just like with GPUs, FPGAs with bigger die areas cost more. The biggest ones are expensive, just like with GPUs.
I suppose the price is a bit misleading because the AX also requires the $12 programmer so with shipping+tax to CA it's closer to $30 total out of pocket.
Personally I use the BX because having the flash, and a bit more logic, is convenient.
Do look for iCESugar on aliexpress instead.