My silencers only cost what it takes to buy some tubing and bar stock. Inconel and titanium are the most expensive. Stainless is not cheap either. I use mostly 4130 and aluminum for mine. Aluminum is the cheapest and just fine for pistol and subsonic rifle rounds. I also spent $1600 on a lathe which was the cheapest one I could find with a 1.125" spindle bore to allow me to easily threads most rifle barrels. Steel is required for the higher powered rifles. It is also a good idea to make the first baffle from steel even if it is going to be used for a semi-auto 22lr pistol. Steel resists erosion much better than aluminum.
For those here in the USA, silencers are legal for unlicensed civilians in AL, AR, AK, AZ, CO, CT, FL, GA, ID, IN, KS, KY, LA, ME, MD, MO (with C&R), MS, MT, NE, NV, NH, NM, NC, ND, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WA, WV, WI, and WY. Two good sources of info are
http://www.silencertests.com/links.htm and
http://www.subguns.com .
Ranb