I like all of these beds, but they certainly vary in price.  The hammock will always be the cheapest, but the alternatives are unique.  This is the Le Beanock, which seems to be only available in the UK:This The Floating Bed, for which you can get all kinds of accessories like tents that hang on the outside and memory foam and sheepskins:
This is really my favorite, the Brazilian hammock.  Cheap and comfy:

We are trying to replace our queen size regular bed with a king size ecological alternative, but it's been tough to find.  We are not big fans of futon mattresses, and although we would compromise with latex foam on the top, it's very difficult to find a king size futon frame that works decently.  We live in small quarters and it would take up the entire room so something foldable or portable yet still huge would be ideal.  It may be that we may end up with a thinner Japanese futon that is easier to fold out of the way, and a tatami mat might be the way we will go.  Maybe we'll do a Japanese thing through the whole house now that we can start over with all of our furniture.  This is how they are stored away (you can see the mats on the floor):