Not sure if anyone did this already (I couldn't find it), so I've decided to share, since I've made for myself anyway:
It shows how much gold you spend for each statue xp point, plus how much gold for each unit of reward. For example:
(There's also another tab to help you keep track of redeemed rewards on a given week)
EDIT: Note that, in my country, as far as numbers are concerned, we use dots as Americans use commas and vice versa, so the dots you see in the image are separating thousands.