In the US, in the Chinese & Asian food markets (probably elsehwere too), you can by Alum which is used in something related to food. In other words, it's cheap and relatively safe. It is also used in styptic pencils which stop bleeding.
Chrome alum, now I don't know if it's identical to alum, but only costs $37.95 per pound in the US. In other words, the price you were quoted is ridiculous!
I'm not sure if grocery store alum can be used as a hardener, but there might be some discussion about this on APUG and it's worth exploring.
edit: Ahh, here you go, "If it's for food use, it's most likely potassium aluminum sulphate - not 'chrome' alum.
K Alum will harden gelatin, but not to the extent that chrome does. Both perform better with the addition of some acid." from http://www.apug.org/forums/forum42/8...e-recipes.html