I generally figure on paying no more than $25 per keg plus shipping. These fall right into that category. When I first started kegging I could get them for $15 plus shipping. Damned Chinese!
I also watch Craig's list, thrift stores, yard sales, and let all my friends know I am looking for kegs. Not too long ago I got a couple kegs for free from an electrician friend who was doing remodeling work at a bar. The bar owner just wanted someone to haul the kegs away because the Pepsi guy wouldn't take them. There were about a dozen in that lot.
Be patient and ever vigilant and you will find the occasional free/cheap keg.
Wayne
Bugeater Brewing Company