Hi NoGrey!
Regarding alternatives to the $100 Honda petcock:
I installed a K&L brand #18-4161 petcock on my '99 Interstate last September.
link to K&L's website: https://www.klsupply.com/carb-fuel/fuel-petcocks.asp
K&L lists it on their website as for the Valkyrie.
It is a made-in-Japan aftermarket valve that appears to be a close copy of the Honda valve.
It appears to have 2 diaphrams (one for vacuum and the other for fuel) with the safety 'vent to atmosphere' weep hole like the Honda valve.
This is NOT the single-diaphram design of the Chinese copies!
However, I did not take it apart to examine the internal construction.
I found it on Ebay for about $60.
Typical Ebay listing as of 03 Feb 2019:
Link:
https://www.ebay.com/itm/Honda-GL1500-C-CD-CY-Valkyrie-1997-2003-Fuel-Petcock-Gas-Tank-Valve-K-L-18-4161/142418029821?epid=202863331&hash=item2128c6acfd:g:J3QAAOSwYDVZ3m8L:rk:1:pf:0It's been working fine.... so far!
I hope this helps!
Jim