Hum. Did medieval Europe even have 4 tons of gold to play with? Notwithstanding the assumption that any character of the appropriate level has that much wealth to play with I wouldn't assume that devices using that much wealth would be first or second generation magic items. Multiple or even many iterations of wealth generation by lower level devices (say, a command operated Plant Growth item), and spells like Fabricate, Wall of Iron and the Creation spells would be needed first. But cities that manage to spend the wealth for something like a Cure Disease gateway would have a ready-made source of wealth as a pilgrimage point.

Also, bear in mind the medievals were able to build some very extensive structures by amortizing the construction costs over decades. 30,000 gold isn't as unattainable by a city when divided by say, 20 years.