As far as aquiring it... you need to first learn your state's (or country's) laws. Then see a liscensed dealer (Sorry so vague, I'm antsy about telling people I that I don't know anything that could be taken as legal advice
)
The cost depends on what you want from who... the higher the velocity/projectile size, the more you're gonna pay. Also, In My Humble Opinion, stick to well known manufacturers (Surefire,AAC, Gemtech, Knights) don't just buy one that seems priced right.
My AAC was $775 plus tax PLUS BATF $200 tax stamp, and those don't come over night.
Also you need a barrel that is threaded (read extended for most semi autos) and it must be threaded to the proper suppressor that you want. Also most semi autos need a recoil booster to function properly as the suppressor adds weight to your barrel, slowing the guns action down.
Dont believe the movies. There are a few (LIKE 'THE OUTBACK II') SUPPRESSORS THAT MAKE THE GUN VERY QUIET (THESE SOUND LIKE A PELLET GUN ON A .22LR) BUT:
Suppressors are not designed to silence the gunshot....they are designed to make the sound non-distinguishable as a gunshot. Example: Most people will tell you that a large caliber pistol w/suppressor sounds like a car door shutting at a distance.