My local lighting place wanted $24 for 1 mogul socket. I'm not sure exactly what they were trying to sell me for that price, but I ended up getting mine from HelloLights for $6 each.
I paid about $17 for the ones I got at Olympia Lighting, so hellolights.com is cheaper even with the $7.50 shipping.
I made my own brackets from sheetmetal from work.