Ebay has surplus military masks for cheap. Some are junk, too old and useless, some are not. Mine is in excellent. It is drager m-65 gas mask in unused condition. The seal around my face is tight and the straps and everything function well and show no signs of aging. Maybe I got lucky on that one though. The 40mm filters are what is expensive, those will run as much as a used mask and much much over. I think my filter was nearly $30 for a NBC filter with a 12 year sealed life. That was after looking for months and months though. I wouldn't recommend getting filters on ebay, you need to know what to look for, and many make claims with faulty expiration dates or just don't know what they have. DO NOT use old gp-5 filters found on old surplus masks though for anything! Ironically, They contain asbestos in them. A new modern gas mask is unnecessary for around the home and even tear gas in my opinion. They will run much more money too. You're better off getting a pair of goggles and a respirator and save yourself some money too. A respirator should have no problem with that stuff. That setup will work fine for almost anything you might encounter in public or at home. What it won't do is protect against heavy chemicals designed to cause harm and of course radiation. Only thing I can think of with tear gas with goggles is the prolonged exposure will likely eventually leak inside depending on what you get. Protesters seem to get away with wearing a setup like that though without an issue. If I knew I was going to be exposed to that type of thing for hours, I would steer towards a gas mask though myself.
As soon as a gas mask is opened it will start to age out, so once you open it you ideally want to seal it back up (even if in a bag). I think trying to protect against weaponized chemicals and also against regular household stuff is probably best done as two pieces of equipment, because even the best household/industrial cartridge won't protect against certain chemicals.
As far as household stuff is concerned, I finally bit the bullet this year and bought a full face respirator and my single regret is that I sat on the fence on this for so long. Compared to a half face with protective goggles there is no comparison! I got the 3M one off amazon for a hundred bucks. With the full face I now have total field of view when protecting my eyes, complete protection against dust, and the filter I got also keeps all smells away. I can spray paint in my garage and literally not smell a dang thing. It is amazing. Get one!! It seals against my face a million times better than a half-face respirator, even with a light beard! No air gets through this thing except through the filter.
I did the 3M 6800 and 60926 filter. I'll be damned if I can find a great matrix of this filter vs the others, but it is the one to get. It covers the most stuff (particulates and then a bunch of gases). If you have some strange isoteric need there are some other filters, but as a catch-all it's the way to go. Put some ear plugs in and don this mask and you can go and work on anything you want.
I'm glad you asked the question. Too many people think absolutely nothing about sucking down dry concrete, sanding tables, paint, whatever without a bit of protection. Maybe glasses at the most while they suck all the crap down. Masks are much cheaper than doctor visits and lung cancer diagnoses.
3M is always good. Watch out though for sellers on Amazon. Some of those fucking cocksuckers will deface the date of production on the package - hiding old inventory. Ask me how I know. If the price is too good to be true...