The only way to know if the gun you are looking at was shipped with the brass g/f,, is via a factory letter. That $10 letter can be worth a lot if a gun letters,,, AND is a hard to find model. In the .357 Blackhawks,, with less than 800 total that letter,, (there are 2 barrel lengths,) a letter can raise the value considerably.
Since Ruger sold a lot of brass grip frames as an accessory,, it's easy to add the g/f. I personally wouldn't pay a premium unless it had the letter to accompany the gun. And a premium would be a price above what a normal gun would cost. (A normal gun pricing would be the gun, and adding about $200 for the brass. That'd be normal. A premium would be above that.)
Example;
A normal minty .357 Blackhawk of the correct vintage to have a brass g/f; About $550-$600.
An added factory brass g/f will cost about $300.
Deduct about $100 for the missing original g/f.
So the gun w/o a letter,, about $750-$800.
A box,, depending upon condition; about $100.
Now,, a lettered brassie; about $1600.00. And the box,, add about $100.
A premium of about double the normal values.