Please bear with this post; there is a direct tie to the roadmap for future planetary missions.
The Associated Press has an article on how frequently NASA's projects overrun their budgets. "Congress' financial watchdog, the Government Accountability Office, reviewed NASA's newest big-money projects and found most were either over budget, late or both."
Overruns have consequences, "Historically, overruns have caused NASA to run low on money, forcing it to shelve or delay other projects. Often, the agency just asks taxpayers for more money... 'The costs of badly run NASA projects are paid for with cutbacks or delays in NASA projects that didn't go over budget,' Stern wrote [in a separate opinion piece quoted in the Associated Press story]. 'Hence the guilty are rewarded and the innocent are punished.' "
It's easy to pick on NASA for poor cost estimations and the follow-on cost overruns. However, these seems to be endemic to technology development, whether in NASA, NOAA, the Defense Department, or ESA. Overruns, usually expressed as schedule slips, are rampant in the private sector, too. It one time in my career, I worked for a high tech Fortune 500 company, and was responsible for planning roadmaps of products with development costs in the hundreds of millions to billions of dollars. Schedule slips were so common, that I always doubled the quoted development cost (and halved the projected revenue -- sales forecasting was equally problematic) to see if the project still looked interesting.
I cannot offer an opinion on whether NASA is better or worse at estimating and managing costs than other agencies and companies that engage in high technology development. I simply have no knowledge one way or another. That also isn't the subject area of this blog.
Cost overruns, in my experience, can be minimized when projects are simple extensions of previous projects. Much of the technology has been developed, and the engineering teams have lots of experience. That also minimizes the aggressiveness of your missions or products.
What makes this article relevant to future planetary exploration is how to balance risk and predictability in planning a program of planetary exploration. I will be very interested to see how the Decadal Survey in progress balances low-risk-of-overrun missions with high-risk-of-overrun missions in its list of priority missions.