I'll say this flat out .. liability. 99% of bodyshops will refuse to touch this type of work simply because of the liability issue. If cost is an issue, you need to stop right now. Safety is the first and only issue. If your frame is rusted through forward of the rear wheel, it's neither operational nor serviceable. As Black Top said .. if it's rusted through in that location, you've lost a lot of strength to rust in other places too. You don't measure metal strength with an eyeball. And if you're caught with a frame patch and don't have a welder's certification to prove it was done right, you're looking at a world of trouble. Depending on your location, possibly even jail time. Not kidding on that ... I have that first hand from investigating the legality of frame repair on a Jeep YJ I had. I could weld on it all I like ... for off road use. Any frame repair (anywhere) would need to pass a DOT inspection for road worthiness. They won't even perform the inspection without proof of certification on the welder's part.
There's a reason replacement frames cost so much ... the liability insurance and certification requirements.
And a word of advise from someone who'd been there. If your frame is rusted that badly ... your hard brake lines aren't far behind.