Like others, I skipped the topper and just replaced the entire mattress. We bought a 10" memory foam (Overstock, Walmart, Costco, Amazon) for about $300. Its not as nice as the memory foam at home, but we don't sleep on it as often so it should last a while still.
We like memory foam and it performs so much better with a solid base under it, rather than a poor quality coil mattress. Besides, we don't want to deal with the topper shifting under the mattress, etc.
The great thing about memory foam is that its easy to cut if you have a short bed or need rounded corners or anything else.