It's a good question but not one that can be answered easily because it depends on the type of foam. For example Tempurpedic mattresses utilize 5lb density foams which are heavy and brands like Bed In A Box use 3lb foams. Also. The type and amount of support foam can be a factor as well. Traditionally though memory foam mattresses can be heavy

King size memory foam mattresses are heavy and difficult to move around, takes 2 or 3 really strong men to handle it. I am female, 5'4", 180 lbs and 53 years of age. I can put a King size organic latex mattress in my Honda Sonata and deliver it to my customer along with a King size organic fleece and two shredded rubber pillows. No need to flip or rotate our mattresses either like you would need to do on a memory foam mattress.

It all depends on the density on the memory foam used in the mattress. This will vary from mattress to mattress.

1)Honda doesn't make a Sonata. 2) High quality latex is much heavier than even the 'best' memory foam. Tempur-Pedic fools people with the super dense cores.