In 2016 that would be a E31 core, user-specified amount of data SRAM and icache (also SRAM), XIP from external SPI flash, user-specified number of GPIOs. Possibly integrating some simple customer peripheral IP as a memory-mapped device, or MAYBE with a simple custom instruction as a functional unit, SiFive doing a little NRE on that, and doing a $30k 180nm shuttle run giving ~300 chips.

If you could give SiFive your desired peripheral or custom instruction already integrated with Rocket and working on an FPGA (Arty) then you'd get it for under $100k for sure -- if you made SiFive do the work it would rapidly get to be more.

Disclaimer: I was an early customer for the HiFive1 (December 2016) and then worked at SiFive from early 2018 to early 2020, but I don't speak for them.

It's worth noting that if you're interested in making your own custom silicon, you can get it done quite inexpensively. It's not a fast process, but if you want to try your hand at building something that is all you, you can!

This isn't want SiFive was doing - they're providing engineering expertise. Here you're allowed to put together all your own logic gates into... a thing.

https://tinytapeout.com/

I took a look at your blog[0]. You aren't going to need a custom CPU. You'll be much better off with something that's already in production, in the market, well-documented and well-tested. Also it sounds like you have a pretty steep learning curve ahead of you.

[0] https://flyingcarcomputer.com/posts/a-new-personal-computer/

Optimistically tagging on here as it's a similar sort of question.

Say one lone developer gets a bit carried away with verilog and ends up with a description for a chip. I know there's an odd lot style of thing where you can get your chip drawn on a wafer along with a load of other chips from other people. There's probably a way of getting someone who knows what they're doing to attach wires to it, wrap it in plastic or whatever else is involved in "packaging".

Where does one get started with that, and what's the ballpark cost? I'm assuming a fair amount of the OP's $100k is SiFive labour, but I don't know how to guess whether a half dozen custom chips in some sort of packaging is of the order of 10s of dollars or 10s of thousands.

(edit: I've done a lot of software near hardware and have a vague idea that uploading the data to tsmc was called "tape out" and involved quite a lot of money, but I also vaguely remember talking to someone at a conference who had chips made as a hobby so there are some pieces missing from my mental model)