These posts deal with quality, scope reduction, and choices we have to make every day.
On this topic, I recommend a post by Mishking in his Agile Advice blog: Technical Debt .
Let me paste here just a part of his post:
Technical debt is a little different than financial debt. […]
There is no way to know the interest rate on a defect. Part of the cost of a defect is obvious: how much time and materials will it take to repair the immediate problem. (Although even that is often hard to measure.) But there are also lots of non-obvious and probably non-measurable costs. How much effort will it take to get to the root cause of the defect so that it doesn’t occur a second time? How much will it affect our “goodwill” and thus reduce further and repeat sales? How much will the existence of one defect hide the existence of other defects (with their own costs)? How much will the defect demoralize the team and increase staff turnover or reduce productivity? How much of an opportunity will the defect create for competitors? How much will the defect increase maintenance and support costs?