The term ‘gas mask’ can be used to describe a wide range of protective masks. Protective masks come in a variety of materials and features, depending on the threat they are protecting against.
You can spend a lot to get the best protection against every possible threat. I don't believe my risk or exposure is high enough to justify spending a lot on top-of-the-line gas masks.
You can rather buy gask masks at best prices online that are made of high-quality materials to protect against various chemical and biological threats. The purpose of more exotic materials is to improve protection against a variety of chemical weapons.
Chemical weapons protection is more than just a mask and filter. Protecting your entire body against chemical weapons requires special clothing, materials, and detection equipment. Personally, I don’t feel that the level of risk warrants such an investment to protect against each of a dozen chemical agents, most of which would kill you before you wear the equipment necessary to protect against it.
Radiological and nuclear threats are a concern of mine. There is always the possibility of nuclear war. While nuclear energy has a remarkable safety record and future reactors could be much safer than other forms of energy, I can't ignore the danger of great destruction and the inherent fragility of complex man-made systems as well as the possibility of a cascading disaster.
When is it a good idea to buy a gas mask?
Not every survivalist needs a gas mask. First, ensure you have security, signaling, and medical, food, and shelter. After you've got the basics covered, it's time for you to look at more specific threats such as natural disasters, pandemics, EMP, and Chemical, Biological Radiological, Nuclear, and Explosives. This is where gas masks are useful.
Do not go into debt for emergency preparedness either. Save up and pay for emergency preparations with cash, gas masks included.