If it is only 1/4" thick, why even trim it ?  Most will melt away.  You don't want to remove all fat.
I've never been concerned about weight after trimming, but If I was, I'd just weigh the fat and subtract from 15.

My cooks for brisket and pork shoulder last year trimmed back almost all of the exterior fat.  I wanted more rub on meat that I will actually keep which is why I trimmed aggressively.  Both cuts have a lot of fat on the interior to keep things moist.

Hard to know without knowing the exact trim you are going to do.  You will have almost no trim on top.  Bottom could be as much as 2-3lbs.  If you are going to take the deckle fat out, that could be another pound.  Trimming down to a 1/4 inch of fat, I could see as much as 4lbs. being removed.

I bought a Costco Prime Brisket 15 lbs.  How much will it weigh after trimmed down to cook?  (1/4" fat on one side only)

If it is only 1/4" thick, why even trim it ?  Most will melt away.  You don't want to remove all fat.

I know that this is unconventional, but when prepping a packer I separate the flat from the point and remove all visible fat from all sides. My thinking is that more meat is exposed to seasoning and smoke. Of course I only smoke to an IT of 120-130°, then sous vide at 132° for 48 hours. Result, Medium rare brisket.

Fredericksburg, VA.  $17.99, $10.39, $5.29, $4.29, $3.89, $5.99

Those are pretty much the same as we have at H-E-B.

Sam's Club increased their prime ribeye steaks by $2 up to $17.98 a pound.