It's not worth it but it can certainly be done.
It honestly wouldn't be that difficult to write a program that looped through a big set of private proxies, clicking on ads while you use cheap services to send junk traffic to the site.
I'd guess something like no more than one ad click per proxy per 60 days. So assume you had a moderately high paying keyword you wanted to target. Something that would earn you like $3/click.
First, the site would have to be legit. It would have to look good enough and have enough good content and a decent back link profile so that under manual review, everything would look normal. You should also be getting at least a good 20% of your total traffic from Google otherwise it's going to look strange that you're getting all this traffic and none of it from them.
The bot could simulate multiple types of browsers, OSs, screen resolutions, etc. It's all just simple info that is pulled via JavaScript or sent via HTTP request headers. The bot would be a decent bit of work but not too complex, it just has to cover all the bases. It would also have to simulate navigating around the advertiser's site a bit for good measure. You could do something like this with Xennoposter worked into a custom app so you can override the OS, screen res and other computer details being sent back from the AdSense JavaScript.
Basically, a properly designed bot, enough proxies and a legit looking site that actually ranks in Google as well as junk traffic to keep your CTR reasonable and you could probably squeeze a few $3 clicks per day out of a site with under 200 proxies which would probably cost you around $60-80/month. So you'd spend let's say $75 on proxies, another $20 for junk traffic sources and $7 on cheap hosting and earn $270 from AdSense for a profit of $168.
Have multiple AdSense accounts under different names, one for each site and with say 5 sites you could be pulling in a $840 a month.
This isn't taking into account the cost/time of developing the bot, the sites and setting up multiple AdSense accounts.
Even if you could triple the number of $3 clicks per day (and triple the number of proxies you need and junk traffic), you're still only looking at around $2620 a month. It's not nothing but again, building the bot wouldn't be cheap and you'd still have to watch out for the fact that none of your ad clicks resulted in any sales. Although at this low volume of clicks, it would be a while before it raised any eyebrows.
Bottom line: it's a ton of work to get going and Google could pull the rug out from under you at any time. You'd be better off building legit sites that provide long term income and can be sold for something if need be.
