Cheapest way to fix this?

Not sure the best way to handle this. I could just sand the rust off and spray primer down and throw a bed mat on and call it or do a full on raptor liner. Any other ideas you guys might have?

 

I would sand/wheel brush the surface rust off, apply a rust converter, sand smooth, and spray with flat black paint or bed liner. You may want to check the underside of the bed also for rust areas.

 

Put some nail polish on it.

 

Ya can’t just put primer down. It absorbs moisture and it’s not a finish. Scuff it, sand it if you want,
Primer then a coat of paint. It’s a truck bed, so any durable auto paint in colour of your choice.
Bed liner you’d have to scuff the whole inner bed and add primer over it all before spraying bedliner.
By that picture that paints just been polished off. I wouldn’t look any further for rust.