Thanks for the tip on the Kohls toppers (btw, Kohls is ALWAYS having sales, coupons).
A topper won't 'fill in' your sagging spots. So you need a mattress.
My understanding is that the main (only?) advantage of a topper is it softens a firm mattress. My last mattress was a waterbed and my current one is a 20 yr foam mattress. It is/was much firmer than I was use to, which is why I got a topper.
I'm in the market for a mattress and/or topper (I need to sleep on the mattress w/o my topper to tell).
The best deal I've found on mattresses is: Sams Club online (thanks to Clark Howard.com). Quite cheap, respectable reviews (for the price). I'm going to become a member, buy the mattress, cancel membership and get a refund (once I know I like the mattress).
I don't think it will help, but I've often wondered - can you cut the pillow 'top' off a mattress to flip it? It may not look pretty, but you're going to put that side on the bottom, so who cares...