Having just spent a tedious amount of time placing coin on several to-be built objects on a plot, I have this suggestion...

Instead of '0' - when going to place coin on a resource to be applied, let the default number be the remaining units to be placed...something that is already annotated in the managing window. That alone would significantly enhance the functionality and ease of making paid work available. Then all one has to do is adjust the coin per unit and if one only wants to pay for a set number of units they can adjust it accordingly.

Aaelefein