Car Cancer. On a Saturn?!
 

I've mentioned this before, but I had my camera with me so here's the rust spot on my car. It's amazing how fast this spot developed. Maybe it was rusting from inside out and I didn't see it.

Earlier in December, I took it to a body shop and asked an estimate on a repair because I don't want the car breaking in half.

Guy said, "Well, I gotta remove the panels, and cut out the affected metal, weld in new stuff and then paint it." Yeah, I knew that.

"You're looking at probably close to $2000." Oh. Is that all?

"I'm not guaranteeing it'll stop the rust either." Thanks for your time, gotta go!

"200 thousand miles? Looks real good!" Thanks for nothing!

My 94 has a spot a little worse than that on the back passenger side. Same spot. I'm guessing water collects in the door jam. It has been like that for about 5 years. Hasn't broken in half yet. You will be fine, maybe try to grind off the rust and spray some rust inhibitor in there.

Drill a hole in it and get some Tuff Stuff foam sealer and fill it up! Maybe that'll keep it from getting worse.

i would stay away from spray foam at all costs, or if your considering it, make sure you get the stuff rated to keep things water tight! We did that in my brothers toyota and it made it worse. we found out with enough exposure it would become porous and start to act as a sponge. we found out how bad it had gotten when he 'found' that tree in the woods.

I would suggest Por-15 as it uses the rot thats actually there to seal it up good, just a light scrubbing needed (no need to take all the way down to bare metal) Its alittle bit more on the expensive side but it works wonders. Ive used it on my truck and has kept the (not repaired) areas of the frame from getting any worse. I would take a funnel and pour some into the hole you already have there, then slather some on the outside.
Check where your rear trailing arm mates up to the car. Chances are if its that bad inside the car (im assuming rear door) you will have some rot there too.

If you want to repair it yourself, i would go find a donor and cut the good section out of the donor (after you figure out what areas are affected in yours) and make patches out of the donors unibody so they match perfectly. but if it is at the trailing arm hookup too, chances are your on borrowed time. sometimes its just too much work to make it justifiable

I was being facetious about the Tuff Stuff. I don't even like using it on wood applications for the reasons that Octavious stated.