Which is the cheapest plastic that retains its structure at 85 °C or higher temperature? In an academic project, I need to protect some electronics components from heat (around 85 °C). I am searching for both a cheap plastic film and a cheap plastic to be used in a injection molding process.

As matt_black referred, the main criteria of the selection is "the point where structural integrity is reduced by repeated use at that temperature or the point where the polymer starts to creep".

By now I am using cork, but I would like to use a more robust material.

Those materials should be both thermal and electrical insulators and should not absorb water. They should also be resistant to the external environment: rain, sun (UV radiation), dust, etc.

Which materials are the best option in this case? From what I have searched, these are the main options in terms of thermal requirements:


PPO/PPE based
Polypropylene
Polyurethane
Vinyls
Polybutylene
Acetals
ABS and SAN
Polystyrene
ABS/Polycarbonate alloy
Acrylics
Cellulosics
Polyethylene and copolymers
The melting point is not going to give you the answer you want.  To properly select a polymer you need to look at the glass transition temperature $(T_\mathrm{g})$.  This is the temperature at which a material loses crystallinity and starts to more easily flow.  This parameter varies widely with thermal history and internal stress of a polymer material and can be raised by annealing.  

Based on UV resistance, I would exclude any olefin or vinyl polymers as well as ABS.

For water resistance, I would exclude cellulose polymers and acetals.

Based on the temperature requirement I would say polystyrene ($T_\mathrm{g} = \pu{100^\circ C}$), acrylics $(T_\mathrm{g} = 85 - \pu{165^\circ C})$, polycarbonate ($T_\mathrm{g} = \pu{135^\circ C}$), or PPE/PPO ($T_\mathrm{g} = \pu{215^\circ C}$) have sufficiently high $T_\mathrm{g}$'s to remain as acceptable for the mentioned specifications.