While security experts think the zero-day may be overpriced, they think the hacker will find a buyer regardless.
So by definition they do not think it's overpriced.
Here is how Israel does it, does it for free and very effectively: they let the screening agents to pick and choose any passenger that they want or have a hunch.
Yeah but errr... they're not private. What's your argument? We should privatise? OR we should do it like Israel? To do it like Israel will cost billions of dollars of public money because... the Israelis hire they're smart people and give them a great deal of training. Your average TSA grunt is so dumb they would have died off before reaching breeding age if they were born more than a few generations ago.
I don't know if any space faring alien society would necessarily be evil.
Evil is a human concept, it does not apply to aliens. Take morals out of any deals with aliens, we have no idea if they would even have such concepts, and if they did they would almost certainly be so different to ours as to make conjecture at this point useless.
But you can consider some more basic concepts that might have a bearing on the actions that an alien species might take upon discovering the earth.
Consider that to be a civilization capable of interstellar travel, they would almost certainly have a good understanding of math, physics, risk mitigation and many more.
Risk mitigation is the interesting one. If we met a warlike species in possession of a planet, who show practical propensity to absorb our technologies and become more like us would we:
1] Wipe them out before they are a threat to us.
2] Allow them to grow and progress to the point where they could wipe us out.
Answers are interesting - I lean very much towards 
Waste not, get your budget cut next year.