The rust hole may look little to you, but further inspection (up on a lift) might tell a different and expensive story.  I'd take it to a muffler shop for evaluation.  They weld everything.

Usually visible rust is only the beginning and there is much more extensive damage hidden. I'd do a hammer test on the area around the rust hole and the rest of the unibody frame rails to get a better idea of what you're dealing with.

Rust is never as small as it looks. I found a pair of tiny rust holes in my Ranger's frame the other day after noticing a sight discoloration and pitting in the frame rail. I whacked it as hard as I could with a 20 oz framing hammer and rust chunks fell out, opening a hole half the size of my pinky pad or so. As others suggested, wince a little, pound it with a hammer and hope for the best. Rust on a unibody is a lot harder to fix than a body-on-frame vehicle like a truck.

Hard to say without seeing the rest of your vehicle.

Bang on it with a big hammer and see if pieces fall off like Scotty Says. He has a number of very entertaining videos on the subject.

Even if you verify it's solid enough, personally I would not spend the $$ to fix it.

I would however use a rust inhibitor spray can to hose down the inside and outside of the hole.

That may hold the rust at bay for a few more years.