Beds have a slatted frame which can add a certain amount of springiness and can get extremely expensive.
Mine is extremely stiff and does not add any springiness at all and I don't have any problem with it so in my opinion you don't need super expensive beds or mattresses.
On the other hand a bed is something that you could keep for your whole life so getting some cheap shit that starts to creak and come apart after ans year is no fun either.