I started borrowing my brother's 3M respirator.  I got sick of using his so I went on Amazon and found the same thing without the 3M name for $25.  They've gone up but still a lot cheaper than 3M.  It looks identical and so far I haven't noticed it falling apart or leaking or anything.

Filters are easily changed/replaced so you can use the cheap dust filters for dust and swap in a gas/vapor filter for NH3