In the end I didn't manage to make my 30 hubs - only made 17. These 17 hubs produced $11.77 in Adsense income.
Looking closely at the results, all the income came from just 7 hubs. Clearly only some niches have readers inclined to click ads - the others attract readers too savvy to click ads at all. Of the hubs that got clicks, some got low value clicks, but the click rate was high, while others got high-value clicks, but the clickthrough rate was low. Obviously the best thing would be a high clickthrough rate, a high value click and high traffic, but I'm not complaining. As long as I make some money from them, I'm not bothered.
What I really want to do is avoid the niches where you put in a lot of work writing a good useful hub and earn nothing from it, and I now have an idea of where to concentrate my efforts.
The other thing I noticed is that it's worth writing really useful hubs - some hubs I produced got found in the search engines straight away (because hubpages gives a lot of juice to new hubs) and then attracted natural backlinks from other blogs and sites. I was very excited about that, especially as these hubs then went on to get a lot of traffic and produce income. Given that getting backlinks is the hardest part of earning on the net, it was interesting to learn how to get these spontaneously.
All-in-all I'd say it was a very useful experiment. I intend to continue writing hubs till I've got about 100, and will be focusing on those niches I feel are both lurative and which I can write something genuinely useful that isn't present on other sites.