Yours does not look to bad, compared to mine. Are there any holes, or is it mostly surface rust?
If it is not rusted through, I would not replace the trunk pan. Grind out as much rust as you can with a wire wheel, and then treat the rusted areas with Rust Mort, or rust removal gel, then primer and repaint.
If it was swiss cheese like mine, the a trunk plan replacement is the way to go.