We started this project over a year ago. The first pad we purchased was a queen-sized 2" version that we bought for about $200. The prices have dropped quite a bit since then.
The 2" foam made a significant difference in the comfort of the futon mattress. The form-fitting property of the memory foam is something which must be felt to be understood...it is kind of like floating without the rolling effects. Another thing we noticed was we didn't need to get bigger sheets to cover both the foam and the mattress.
We purchased the 4" foam version from Overstock.com. They had great prices and you gotta love the $2.95 shipping! Amazon also have great prices on these foam toppers. The prices change all the time, so check around for significant discounts.
A 2" pad is probably sufficient for anyone less than 150 lbs. The 4" pad might be a better choice for weight heavier than 150. There is a noticeable difference between the 2" and 4" mattress toppers, although that difference could be from the density (4 pounds vs. we don't know what) rather than the height (4 inches vs. 2 inches). However, we do think a major increase in comfort factor with the 4" topper is its ability to block motion transfer. This is especially important if you or your partner has a more, um, active sleep pattern than yourself (not that I'm noticing)...
In order to demonstrate this point, we have captured a few short clips to show the 4" foam's ability to absorb movement. A traditional spring mattress cannot even hope to be able to compete in this area.
We used a three pound weight to illustrate movement. The first clip is the famous wine glass test...only we didn't want the interns spilling wine everywhere :-) We came up with a better way to illustrate the concept.
Click on the video clips to see what we are talking about. The center video shows the water shaking when the movement is close by. The shaking movement is quickly dissipated. When the distance was increased to just a foot, there was hardly a ripple in the bottle of water!
We found the memory foam topper to be much more comfortable than just the bare futon; this is especially true of the one of us with the curves. We have not yet tried a full memory foam mattress, so we cannot compare the topper to the "real thing." We would love to hear from readers who have experienced the "expensive" versions...is it everything they say it is?