It all depends on how DIY the OP is and how long he can have his truck down. I would say, if the OP hasn't considered fixing it, and doesn't own a welder, then he's probably doesn't have the DIY mindset (for metalworking). Maybe they have other DIY interests instead. Most of my neighbors, good builders, landscapers, whatever ...but I wouldn't want to own a truck they did any fab work on.
Metalworking ...the place to begin with metal fabrication for a guy who's never touched a welder isn't his truck's rusted gas tank frame. And depending on what the rust damage looks like, it could take a lot longer than one at first thinks (esp for a non-experienced guy).
Besides a welder, the OP would need a number of other tools to do a 'good' quality job. When I get into a real fab job ....sometimes I look around at the beavy of air and electric tools, grinders, torch, pliers/vise grips, different hammers, you name it. All the tools a bodyshop would use in the normal course of a day. A welder just gets you the stitching tool. To do a good job that won't rust out again in 3 years, a fabricator needs other tools too. The repair job could take a beginner all day to do and could be really discouraging on a hot mosquito day. I say ...just pay the $650 and be done with it.
Most of my neighbors and people I know aren't DIY people beyond changing their lawn mower blades (which they don't even sharpen) .lol. And there's nothing wrong with that. Their Fantasy Football knowledge is far greater than mine