As for the other economic incentive, you would need to compare the potential saving with the cost to the state of keeping someone in jail, which can run anywhere from about $15k (Texas) to $35k (Maine). Weather and geography are significant factors in this variation. The basic income we're talking about is probably only sufficient for basic survival; somehow I doubt it's going to be more than the cost of imprisoning someone.
Of all the possible unforeseen ramifications of how something like this might work out, putting people in jail to save money is not high on my list of things to worry about...though now I think about it, the cost per inmate per year might be a reasonable metric for determining this basic income, since it reflects the approximate cost of living in the most literal sense.
I just finished reading through the Self-Enforcing Protocols post (http://news.ycombinator.com/item?id=755131), and this scheme strikes me as something being easy to abuse.
Um, maybe having the kids around?
Separatist cults would now be profitable instead of being money sinks. Debtor's prison is now a possibility again (you can house the person more cheaply than their annual stipend).
If the government gets to confiscate the stipend of incarcerated felons to offset prison expenses, there's now economic incentive to increase prison population.