"When we have a political discussion about any other government agency, there are usually some kind of stats we can judge our expenditures against"

Usually those stats are ignored. You brought up good examples:

"Need more police? Maybe so. What's the crime rate look like"

More like, "I can be tough on crime, so let me fund more cops!" The crime rate has been declining lately, but the police are more powerful than ever before. Paramilitary teams are routinely used to serve search-and-arrest warrants, the police can recycle the proceeds of seized property into their own budgets, and there are more and more ways for us to become criminals with each passing year.

"Need increased defense spending? I don't know. What's the defense spend of other countries"

Once upon a time, we only spent substantial money on the military when we were in a state of war. Then we began to panic over the USSR's military might, so we created a standing army. Now there is no USSR and we have the most powerful and well-funded military on Earth. Despite the changes in other countries' military sizes and budgets, our standing army faces more budget cuts from failures to compromise on the domestic budget than any sort of quantitative comparison with other countries.

The thing about the executive branch is that it is constantly trying to get more power, and the legislative branch is constantly giving more power to the executive. The old protections against executive abuses are being eroded, and new ways for the executive to act without democratic process are being created. Declare drugs to be illegal without consulting Congress? Sure. Shoot and kill American citizens hundreds of miles from any battlefield? Of course! Expanded the military presence in countries that never threatened or attacked us? Why wait for Congress on anything? The only politicians who are even trying to stop this are outsiders, libertarians and the far-left whose ideas are nowhere near the mainstream.

"With the TSA, we're protecting them from something not happening that's only ever happened once in our history."

The issue here is that there was never actually a problem for the TSA to address. America does not have an ongoing problem with terrorist attacks; every few years we see one very determined attacker manage to cause a lot of destruction, and otherwise we go on with our lives oblivious to the numerous ways we can be attacked. Other countries are not so fortunate: they have to deal with terrorists attacking garbage cans, buses, markets, etc. They can judge their policies by reductions in such attacks.

Bruce Schneier has a good point: instead of an agency meant to secure transportation against rare and unpredictable attacks, we should fund an agency meant to make transportation generally safer. We have lots of car accidents each year; once upon a time, we addressed that by mandating seatbelts as a standard feature. Why not continue to develop safer cars? If we are no longer creative enough to do that, why not spend the money on making alternative modes of transportation easier, cheaper, and more available? We can determine the success of such a policy by the reduction in vehicle-related injuries and deaths.