Stick with buying frames from suppliers in winter sales -- they're cheap enough.
Here in NL where I live, frames are not cheaper in winter than in summer. Non-self-spacing frames unassembled will set you back about eur 0.90 per frame, and assembled+wired about eur 1.30 per frame. Frames from dead beekeepers may cost as little as eur 0.50 per frame, but there's great demand, so little supply.
I would think ply would delaminate far too easily and mdf isn't renowned for it's moisture resistance ?
The facts as I'm aware are:
1. The humidity in a hive during brood rearing is 50%-60%. The bees keep the humidity fairly constant, because it affects brood rearing. Of course, during winter the humidity in some parts of the hive may exceed 100%.
2. When you buy MDF/plywood, the ambient humidity will be 10-20%. This means, if you use them in the hive, they will be exposed to a one-time increase in humidity from 10% to 60%.
3. The increase from 10% to 60% humidity will cause the MDF/plywood to thicken about 6%, and lengthen (i.e. in length or width) by about 0.3%. If I understand the science correctly, continuous exposure to such humidity levels do not increase the thickness/length compoundly.
4. The dramatic deteriorisation in MDF/plywood seen when used outside is actually due to contact with water, and not with mere 90% humid air.
http://www.ewp.asn.au/library/downloads/ewpaa_facts_about_pb_and_mdf.pdf
So, in theory, it should be okay.
What attracts me to MDF/plywood is that it's dead flat and available in highly predictable thicknesses. Only very expensive wood is like that.
Samuel