Depends on what percentage your total score comes from income. The penalty is supposed to be 20% although I do believe that I have seen it as high as 40% for very high levels of income (i.e. over 1 million bc's per week).

Anyway "just" a 20% reduction seems to be more than a negligible amount from my point of view, however this certainly doesn't result in a 20% reduction in score since most score increases appear to be logarithmic, that is linear increases in score require exponential increases in income.

However with the above said, in my gigantic games my average game ending income is on the order of 1.3 million bc's per week and in my experience not spending my treasury below 20K results in an income of "only" 800K per week and this diference is indeed a very noticable difference in score. However in these games income is a major component of my score.

Okay, I need to once again voice my disdain at this rule. I hate that i get penalized for having a really strong economy. Oh well, nothings perfect.