Rusty Frame

It sounds expensive to fix.

Add the cost of your truck, the replacement frame plus any potential shipping, and then the actual replacement labor. Also be prepared to factor in potential extra items that may break or may be needed during the swap like suspension, rusted cables or lines, hoses, etc.

Then figure what you may get if you try and either sell it to a recycling center or dealer or part out and find a replacement for this truck. Do the math and see which is better. Factor in down time. The frame swap could be a few weeks depending on frame location.