Silica aerogel of ·1 g/cm3 starts at $8/g or 80¢/cm3. If you'd like densities toward the record of 1 mg/cm3, multiply by--I don't know--500? Carbon aerogels are somewhat cheaper, resorcinol formaldehyde cheaper still. You could make your own and maybe do better.
It seems a 3mg/cm3 aerogel costs $2100/g or $6·2/cm3.