I imagine (but I have no real information on this, so I might be wrong) that cathedrals were always works in progress, on which you could throw a small percentage of your surplus in the good years, and keep idle in the bad ones (or maybe also a bit the opposite, in anti-cyclic fashion). See how the Sagrada Familia is being built in Barcelona (and has been, for 137 years): with an annual construction and maintenance budget of around 25 million, a very small yearly amount for such a huge project.