I was quoted $595 or so for a tub liner. It's plastic, and apparently a little fiddly to remove, so it'd be inclined to sit in there ...
Dirt and moisture would accumulate under that and encourage rust like no man's business, so we opted out. Rhino linings might be the way to go, or if you like DIY, you can get the Rhino Liner spray can for about $130 or so (if memory serves) in Supercheap.
For now, we're not getting ours treated and will use those click-together gym mats cut to shape. Might save dents, scratches *and* movement (slides & rattles) - I'll let you know when we actually get the thing - expecting it late this week or early the week after.
Cheers
Tony