If you'd like to know what silencers REALLY sound like, check out my movies.
http://www.youtube.com/user/elitearbor
As for useful life, most silencers are good for thousands and thousands of rounds, some are rated for over 50k rounds, and a few for more than that!
As for cleaning them, most silencers require little to no maintenance, except for .22lr silencers; these have lead build up in them due to the .22lr bullet being unjacketed.
Inexpensive .22lr silencers start at $90-200, and go on up to $400-$800. The decent starter silencers for .22lr are $250-$350. Centerfire, it depends - pistol or rifle? I have a pistol silencer that was $400 (9mm) and a rifle silencer that was $550 (5.56), but also a pistol silencer that was $750 (.45acp) and a rifle silencer that was $800 (30-06). Prices depend on construction, reduction in sound, etc.
Regarding permits - no. It's a one-time tax stamp for possession of the item, each item is $200 (silencers, MGs, sawed offs) or $5 ("Any Other Weapon" like cane guns). The 1934NFA is easy to obey and just requires a bit of patience while you wait for the tax stamp.