The New Zealand Treasury has conducted a preliminary assessment of a guaranteed minimum income (GMI) in New Zealand[1]. It's worth a read.

It's conclusions were broadly:

* To be cost neutral, it would require a significant increase in taxation rates (to a flat tax of ~45% - 50%).

* Although the Gini coefficient is improved, those who currently receive government support would likely receive less under a GMI scheme, which may have the effect of distributing more money away from those most in need.

A brief discussion of the "broad ranging" effects on efficiency and economic growth is also included.

[1] http://igps.victoria.ac.nz/WelfareWorkingGroup/Downloads/Wor...