Why would you want an ASIC? FPGAs are perfect for this if it's only digital. If you need analogue stuff, that's something for a specialist chip designer. Things are done completely different on a chip then you would with regular components.
You have no resistors and capacitors, only in extremely small values, so things are replaced with current sources and mirrors. That's why you some ICs have a 'reference resistor' pin. From that single current a whole bunch of other currents are derived using current mirrors and different transistor geometries. There is an ebook on the internet on analogue chip design, IIRC written by the 555 designer. Very interesting read.
IC fabrication is so expensive compared to PCBs that prices probably never get down that becomes feasible from low volume stuff. Where low is likely millions
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
Except for specialist applications where there's is no alternative to an ASIC; like cochlear implants, there is a reason the implant costs like €25K and the external behind-the-ear piece about €8K.