Different people will have different responses. The thing that you want to place a lot of attention on is the purpose. If you intend to use the futon as your go-to sleeping bed because you don’t have enough room, then you should consider investing a proper amount of money in a quality mattress.
Budget futon options usually pose a compromise on some of the points mentioned above, which may not be what you want if you are sleeping on this mattress every night.
On the other hand, if it’s just something that you place in your guest room to increase the number of people you can accommodate for an overnight stay, a cheap futon bed sounds like an excellent idea to a lot of people.