To add to that, forums and technical sites are notorious for not converting. If you build up the traffic, the advice I've seen is to start doing CPM or affiliate advertising rather than pay-per-click like adsense. If you can help advise others technical people on a product, they'll likely buy it making affiliate a lot more attractive.
Hm, this sounds interesting. So do you say that these will barely bring any revenue? Should I try some "hot" topic like celebrities and such stuff???
I would find better niches, your gonna have to do a lot of work to make those sites rakn well in the serps.
Don't rely on forums to get you money. If you really want to earn money webmastering, make a real website, bring in traffic, and have good content.
I wouldn't go into 'celeb' sites. Despite getting better conversion rates, your ppc is so low it's not worth the time unless you put in ridiculous amounts of time into your site. If you can provide something new and interesting, it might be a different story.
I am on average $30 per month with 70 to 80 daily visitors. This is good for me without spending much money I got good money.
Still having a day job, doing webmastering as a hobby. Am "only" on about $100 on Adsense since the beginning... Not really having time for it now.