Reddit reviews POR-15 45004 Gloss black Rust Preventive Paint - 1 Quart

Also definitely paint the crap out of the floorboards and the battery area under the rear seat with this stuff...it’s a lifesaver too: https://www.amazon.com/POR-15-45004-Gloss-black-Preventive/dp/B00H2VVL0S

How interested would you be in fixing this yourself? With a few tools and a little elbow grease you could stop the progression of the rust relatively cheaply. If the metal is not too far gone I'd say it could be "fixed" for under $100. By fixed I am referring to stopping the rust from getting worse. So long as the rusted area is not structural, and by your description it is not, as long as there are no holes you are fine to just stop the rust from getting worse. I'd say if you've got access to a few tools, remove the rust and ask a shop to look and see if it should be good to use a rust preventative paint/coating.

As far as repairing the metal itself, the only real fix is to cut it out and weld new metal back on. Which is considerably more expensive, though if you do some of it yourself you could save quite a bit of the cost.

If you just wanted to stop the progression of the rust, use an angle grinder with one of [these] (https://www.amazon.com/DEWALT-DW4910-8-Inch-11-Knotted-020-Inch/dp/B00004RHAL/ref=sr_1_1?ie=UTF8&qid=1469088859&sr=8-1&keywords=wire+cup+brush) to remove all the rust and bring it to bare metal. Then get some [por 15] (https://www.amazon.com/POR-15-45004-Gloss-black-Preventive/dp/B00H2VVL0S/ref=sr_1_1?ie=UTF8&qid=1469088930&sr=8-1&keywords=por+15) and coat the area well. This should be sufficient to stop the rust from spreading. If you went this route I would advise doing dome research and just using this over any accessible rust prone areas you can find, though that would just be extra insurance.