For Relics, this has been the idea floating around in my head for a while:
Change the sale price of the upgraded Relic so that it is more than the sale price of the unupgraded relic plus 1.5k electrum (not a lot more, maybe ~50 or 60). Then, if you do not have the money to upgrade it right away, the choice is yours - do you need the money now, or can you wait to get to 1500 electrum so that you can get extra profit from it?
EDIT: I was too late.