Suppressor ownership is dependent upon state laws. Some states allow it and some do not.
The noise reduction isnt "Hollywood quiet". It brings the centerfire rifle rounds (223/556) down to roughly what a .22LR would sound like. Meter wise its not safe levels of suppression in terms of not wearing ear protection but its light years ahead of not using the suppressor at all and you can shoot without hearing protection and not have the ringing and such you normally would.
The cost is $200 for the tax stamp from the ATF. That is for the suppressor and not the firearm so once you pay for it you can use the suppressor on any firearm. In fact one things guys will do is buy a .30 cal suppressor thats used on 30-06 or 308 rifles and use it on the .223. It doesnt work *quite* as well as a dedicated 223 suppressor but you wont hear the difference for the most part. That allows you to forego buying 2 suppressors and just buy one as a "do all" type suppressor.
I almost went that route of getting a "one size fits all" but the 30 cal cans are quite a bit heavier than the 22 cal cans. It adds a noticeable amount of weight to the end of the gun so in the event I end up walking around much..... I went with the lighter 22 caliber can.
They make cans that are 22 cal rimfire and 22 cal centerfire rated. If you use a rimfire can on a centerfire rifle you'll destroy the can. You can go the other way (centerfire can shooting rimfire ammunition) but it does dirty the can up pretty good.
Suppressor costs are anywhere from $250 - $2000+ depending what you get. Most centerfire cans start around the 5-600 mark and go up with a quickness from there. The rimfire cans are 250-600 or so. Throw in a $200 tax stamp on top.
My can was $550 or so plus $200 on the stamp for around $800 all told with transfer fees and such. Worth it? I think so. I can now shoot while the kids are out playing in the yard whereas before it was so loud they would go inside. I can teach the kids to shoot without requiring hearing protection although if we shoot very much at all I will have them wear it. My hearing isnt so good these days so.....
Lastly its the type of thing that lasts a lifetime. Yes it is a lot of money, but in my opinion money well spent. Its great not needing hearing protection, great not annoying the neighbors and such....Very happy with it.