I’ve heard Duct tape was good for about anything Moots. :)
On a serious note, you’ve made a good first prototype. X:X Now it’s time to consider how to improve the design. It’s rare to get a design perfect in the first revision. I’m on about revision 8 of my foam hives. As Edward says, the bees will eventually start chewing up your Styrofoam but that takes a while. The more immediate problem you’re going to have is your foam frame rests. The bees will glue the frames to the rests and you’ll have one huge mess. In order to remove frames beeks typically break a frame loose on one end and then slide the others to get them out. Neither activity is going to work with foam frame rests. I use wood for my frame rests.
The duct tape on the mating surfaces is thinking in the right direction; just the wrong material. It won’t last and the robbers have a tendency chew into the hive around duct tape. When they find a discontinuity of material, that’s where they start chewing.
Foam is stronger than it might appear as you demonstrated. Here they build houses out of the stuff: http://en.wikipedia.org/wiki/Structural_insulated_panel
I build my hives out of extruded polystyrene that is rated for 15 psi. They end up with a foot print of about 120 square inches which could support up to 1800 lbs (800 Kg).
My suggestion would be to glue the foam sides together with a polyurethane glue instead of the duct tape. You get a very strong bond and it’s waterproof. If you want the hive to last for more than a year or two, you also need to paint (or cover) the surface to protect it from the Sun’s UV rays. I simply paint my exterior with a latex primer and a latex top coat. That is all the exterior needs IMO. Not much more weather proof than painted plastic.
I agree with Edward that the interior of a foam hive needs some kind of cladding, or foil, because the bees will eventually start eating the hive away. I’ve tried plywood and it delaminated. I currently use masonite/hardboard because the stuff holds up pretty good INSIDE the hive and it can be glued securely to foam.
Finally, I hate to admit it, but I would stick with wood for the supers. The supers get a lot of abuse and I really don’t see a great benefit to insulating them since (at least in my case) the insulation provides the most benefit to northern bees in the winter and early spring before the colonies get supers.
As for costs, I don’t know what the costs are in Europe, but shipping big bulky things (like foam hives) is expensive in the USA whereas building things from the home supply store is pretty economical due to the volume of scale and shipping via rail. The trick is finding a design that is sufficiently durable to your tastes and doesn’t require too much labor to make; the materials are really pretty low cost. Certainly lower cost than wood.