What would be a fair price for a 5" pre-27 in 90% condition. It does not have the box and has a lanyard ring on the frame. Magna stocks show moderate wear. Thanks!

Has the hole for the ring punched the serial? That might suggest it was added by a previous owner. If factory original the ring would be interesting as it's not often seen on these.

A 5" post-war 357 is a popular gun (not as popular as the 3.5" models) but 90% won't excite serious collectors.

I would pay $500 for the gun you describe but a seller would probably ask $900.

...of course it would be worthless IF the serial number were obscured and it did not exist elsewhere on the gun. That would be a major violation of federal law.
Well, not "worthless" as the non-frame parts are certainly worth something. Never turn in a whole gun as the feds only consider the mainframe a gun, all else are parts.