How much would a modern gaming pc be worth in the year 1990?

Excluding the outrageous technological & chronological anomalies that would be created by such a foolish action...  A modern gaming PC from today would be PRACTICALLY WORTHLESS in 1990 for multiple reasons.

1) Hardware architecture of the modern system would be 2 bit generations newer than the present. -- PC's in the 1990's were running on 16-bit architecture & modern PC's worth their value are running on 64-bit architecture (4 times larger than in 1990).

It wasn't until around 1995 (w/ Windows 95) when 32-bit architecture started to appear & didn't become standard around 1998 (w/ Windows 98).  64-bit architecture didn't start appearing around 2003 (a few years after Windows XP).

Hardware architecture tends to backwards compatible with the previous generation of hardware & software with a performance hit (as they're only utilizing half the processing "length").  So the 64-bit architecture would backwards compatible with 32-bit hardware & software WHILE 32-bit architecture would be backwards compatible with 16-bit hardware & software.

2) Software of the time would not be able to run without an emulator -- As noted above, with the 2 generation gap in bit architecture & would not run if you didn't have an emulator OR a Virtual Machine (which is just a more advanced emulator) pre-installed prior to time travel.

3) Software of the time would only be able to utilize ONE CPU processor core -- Multi-core processors weren't around until 2001 & didn't become a standardized component (properly utilized by software) until 2010.

Processors in 1990 were single-core processors & generally ran on more brute-force speed.  Processors at the time were only running around 20 Hz & a low-end modern processor runs around 1.0 Ghz or 1,000,000,000 Hz...  which is 50,000,000 times (or a bit more than 2^32 times, in respect to Moore's Law) faster.

3) NO CONSUMERABLE SERVICABLE PARTS -- This is a common trope with Time Travel, especially when going into the past.  If any part of the PC (or the time machine itself) breaks down for whatever reason, you wouldn't be able to get replacement parts NOR would you may have the knowledge manufacture the necessary component itself (since PC components are typically proprietary & riddle with trade secrets).  This would apply to the keyboard & mouse as well.

The USB standards didn't exist before 1996 & wouldn't become a ubiquitous port until around 2001 (w/ Windows XP).  HDMI standards won't exist until 2002 (wouldn't get into TV's before 2004 & wouldn't become ubiquitous until around 2010).

1990 was the age of legacy ports & mostly analog standards.  Keyboards & Mice still utilized PS/2 ports & video was running on Serial / VGA ports.  While your 2020 PC might be able to adapt to 1990 VGA monitor (as modern monitors still support the old analog standard).  While Bluetooth wireless standard existed at the time (it was created in 1989), it wasn't properly usable & ubiquitous until 2005 (w/ Bluetooth v2.0)

WITH THAT SAID...  if the 2020 PC was given to the right people (like Microsoft on the software side...  IBM, Packard Bell, Hewlet-Packard, ect. for the hardware), it could be considered priceless to some degree as they would be able to fashion & potentially replicate the components.  HOWEVER, since manufacturing processes have radically changed between then & now...  it would like be a challenge for engineers back then to make such a quantum leap in technology (causing a temporal anomaly).

I will note that some of the years & technologies are subject to error as I'm taking a CONSUMER STANCE on this hypothetical situation here.