"Some Justifications for the lay person--
1) NASA can be of great benefit for the efficient utilization of earth bound resources and acquisition of knowledge to protect our planet. With better knowledge to create accurate predictive models, we will make more efficient decisions governing such things as climate change policies. That will potentially save orders of magnitude times the current NASA budget in itself. As a side note, exploration of other planets weather systems will be invaluable in creating earth models--modeling is so much more effective when you have more than one data point.

2) If you can think in terms of grandchildren rather than yourself, then you can get behind the resource exploration that is tantemount to automated lewis and clark expeditions. The potential value of the limitless resouces of space to supplement our earth bound ones is worth some long term vision and funding.

3) While the odds are low, the level of catastrophe makes it clear that we must survey our solar system for collision threats and if necessary build systems to avoid them.

4) The obvious everday uses of space such as gps, weather forecasting, resource monitoring, etc deserve a budget,
5) You need a governing regulatory body for all space and satellite companies
There are more but by now the average person has nodded off. "

Point 3 will likely be the driver for manned missions to the Moon.

Point 5 should be under Department of Commerce already, as concerns operations and space debris