Somewhere over the years, the fabric on my stock saddle has worn a hole on the bottom-most corner and now the tension has caused the tear in material to grow. Mostly cosmetic...for now.

Do you guys know if it's more cost effective to try and re-upholster, somehow patch, or replace saddle?

I bought a new seat for my Adventure for $150. Bet you can find a good used one for $75 for you SD. A recover with better foam might run ya $350.
You do know there is a parts for sale section on this site....along with a wanted section .

watching one on the 'bay <$100...TBH at 0.29 posts per day i'm probably the sole registered lurker here and too slow to jump on parts for sale and wanteds never worked out....excuses excuses excuses

Just a suggestion (and probably not cost effective), but I would consider getting a custom seat if you really love the bike. Everyone raves about James Renazco, and there is also a local guy up here that will take your pan and build you your dream saddle:
I have recovered my own seat on a previous bike and it looked like shit, but your mileage may vary.

Bic found a nice leather cover for it. Available from ebay.
Looked very nice and seemed reasonable easy to fit with a good staple gun.
His is for an R but they had for SD also.
As usual search is your friend!

Saddlemen can recover it however you want and probably put some gel in at the same time. Price should be reasonable and they are in California.

I'm sure you can find a local auto upholstery place that can recover it no problem. I had my seat recovered and shaped and it ended up only costing $75. The Superduke seat is not a difficult seat to recover.

I recall bic had that leather recover...but I really like the grip of the stock synthetic there now.

If I go custom it's gonna be pimpin to contrast the modernistic vibe of the bike: alligator with chrome rivets, tassels fook yea

I saw a Ducati the other day with the Louie Vuitton seat cover...

Threw up in my mouth a little bit.

Replace.
They last a long time and are very reasonably priced.