Here's some good info for ya. Got this from OSHA, these are the allowable limits per day:
Noise regulations in several jurisdictions treat impulse noise separately from continuous noise. A common approach is to limit the number of impulses at a given peak pressure over a workday. The exact figures vary slightly, but generally the regulations in which the exchange rate is 5 dB permit 10,000 impulses at a peak pressure level of 120 dB; 1,000 impulses at 130 dB; 100 impulses at 140 dB, and none above 140 dB.
Also, here's a great table of suppressor info:
Silencer Results
Keep in mind this info is a bit outdated. I just ordered a .223 suppressor that reduces the sound 36db. If you look at the table for .223 unsuppressed it's about 165 db. My suppressor should reduce it to 129 db, which means I can safely shoot it 1000 times a day without hearing protection. Some of the silencers listed don't take it below 140 and you need hearing protection with your 'silencer'! Be sure to do some homework or, in my opinion, your silencer could be virtually worthless and would require hearing protection.
Keep in mind that the decibel scale is logarithmic (base 10), which means that a noise of 100 db is twice as loud as 90 db. 90 is twice as loud as 80 db. (which means 100 db is one hundred times as loud as 80 db!)
Here's an estimate for my Glock 21 .45 ACP (I decided not to get it):
**Some firearms need the LID device to help cycle the weapon. 45s some 9mms. They run about 135.00 for the LID device
**Drop in threaded barrels run from 125.00-205.00
**Will need to get taller sights as well. Front and rear run 90.00-190.00 (night sites are more)
**Gem-Tech Blackside silencers run 585.00 in 45 cal.
**Plus $200 tax stamp
**Plus tax
Plus about five to six months of waiting, background check, passport photos, fingerprints in duplicate, sheriff's signature if you are lucky (and most likely a call from the sheriff asking you why you want a silencer), a bunch of paperwork in duplicate, payment up front as soon as you initiate the paperwork, etc..(you get the idea).