Are these paid themes really that much better than what's available out there for free?
I look at it in terms of hours. I use a rather passe paid theme (blanking on the name) for several sites. Genesis (which I haven't used yet, but plan to purchase as soon as I have time to actually act on the purchase) is $60. If you value your time at $1/hour it has to save you 60 hours of effort... but if you value your time at $60, if it saves you one hour, you break even.
I've found that if I see a paid theme and it is close to what I want, meaning it might save me 10 hours...
$60/hour * 10 hours = 600
Genesis cost = 60
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Net value = $540
If, on the other hand, it's a pain to use and takes me an extra ten hours to figure out....