Despite significant investment in research, the lightweight estimation of development effort is still an unsolved problem in software engineering. This study proposes a new, lightweight effort estimation model aimed at iterative development environments, as Agile Processes. The model is based on Radial Basis Functions. It is experimented in two semi-industrial projects conducted using a customized version of Extreme Programming (XP). The results are promising and evidence that the proposed model can be developed incrementally and from scratch for new projects without resorting to historical data.