Clicks are an equally worthless metric.
A friend wasted some $75 on Google Adsense trying to expose a free game we've made to more potential players.
As it's similar (but not too much) to Minecraft Classic, we figured we'd just try to get Google to show an ad to the side of searches for stuff about Minecraft Classic, and some people might decide they're interested and check it out.
As Google presently showed no ads whatsoever for searches for "minecraft classic" and a few other search terms we wanted to display ads next to, we assumed this would cost us almost nothing, that they'd be willing to display them for a penny as we didn't have to out-bid anyone. That couldn't have been further from the truth. The minimum we could get any advertising for was $0.25/click, and none of that was on Google's search results.
Half of the clicks came from random web sites with the most horrible games (as in, there was sound and graphics (ripped off from other games), but no playability whatsoever) which displayed a dozen ads on each page with one of these games. Thus, those clicks were completely worthless to us as they were likely all coming from three year olds who simply didn't know what they were doing. Chances of a three year old downloading a game, installing it, and running it aren't that high, and we weren't interested in three year old players anyway.
The other half came from paid search result placement on altavista.com. These clicks were also completely worthless. Just think about it: You're searching for something related to a game you like, and you get sent to a web site about a completely different game, so what do you do? Do you say "yay, I'll download and play this instead" or do you just immediately click the back button and look for a search result that's related to what you searched for? We specifically wanted ads to the side of people's search results because then they'd know that what's over there isn't necessarily exactly what they're looking for and so they would only click on the ad if they were interested in finding a new game. Paid search placement may increase the number of clicks we get, but it ensures that those additional clicks are as close to worthless as can be imagined. Think about it again: How long would you stick with a search engine that does this? Only morons and five year olds use Altavista. Not our target audience, and if you're trying to sell a legitimate product or service, probably not your target audience either. On the other hand, if you're trying to scam people out of money, paid search result placement might be exactly what you're looking for.
After going through $20 on these worthless clicks, I looked up the statistics of how many of the IP addresses ultimately ended up playing the game. Normally about 50% of the IPs that visit the web site end up downloading our game and 30% (60% of those who download) connect to the game's servers, but not one of the people who clicked these links even downloaded the game.
This is the kind of bullshit you end up with when you consider that clicks are the only important thing: a system optimized to give you clicks at the expense of any sort of quality placement.
Anyway, it seems that if you want results displayed on google.com, you have to pay at least $1 per click. The adsense interface gives you all sorts of BS reasons about why your page won't display ads if you offer less money than that, but it's all designed to make you think there's some legitimate reason you can't figure out until you ultimately give up and just decide to give them more money and see if that fixes the problem. In particular, the supposed metric of the quality of your landing page is almost a random number generator -- just wait a while and it changes, as it's apparently chosen by monkeys. Eventually my friend paid the $1 per click, despite my objections that the whole deal was a huge scam, and as far as I know we got nothing for that either.
The one thing my friend did that did help was to list our game on www.indiedb.com as the people who showed up from there, while only about a dozen each time he's updated our information, were honestly interested in our game, downloaded it, tried it, said it was great, and never played again. (Our game's main issue is that it's just not that interesting.) That didn't cost us a penny and it was the best advertising of all. Needless to say, if our game ever does become interesting and we want a few more people to look at it, we won't bother with Google Adsense again, we'll just go to Indie DB.