is it a must that when your want to upgrade your drug front property that you pay the price as if you were buying it new?
in reality wouldnt the upgrade mean at most you need to add more space to the property? i think there should be different size and cost properties. for example, 
1-5pounds   -  closet
6-15pounds  -  basement
16-30pounds  -  house

those are random ideas of the dome but i think with the help of others this can bloom into a great idea

I like the idea, and maybe the drug front property should be able to hold drugs because dealers aren't going to actually carry all the drugs they have

 Well, to balance this idea out, the bigger the space of the drug front, the more often you must bribe to keep off the raids.

Like every 4 days for a closet, 3 days for a basement, etc.