Liquid fuels (bioethanol and biooil) derived from herbaceous crops are considered beneficial for the environment and human health especially if they are used as fuels for motor vehicles. The choice of the most suited crop to be cultivated for liquid biofuel production depends on many factors; the most important being the economic convenience for farmers to cultivate the new energy crop in place of the traditional ones. In order to analyse the conditions which favour the cultivation and selling of specific energy crops, a simple methodology is proposed, based on the calculation of the ''threshold price'' of the energy crop products. The ''threshold price'' is the minimum price at which the primary products of the energy crop, i.e., roots, tubers, seeds, etc., must be sold in order to obtain a gross margin equal to that usually obtained from the traditional crop which is replaced by the energy crop. As a case-study, this methodology has been applied to twelve Italian provinces where the cultivation of six energy crops, both in productive lands and set-aside lands, is examined. The crops considered are sugar beet, sweet sorghum and topinambour, useful for bioethanol production; and rapeseed, sunflower and soya, which are usually employed for the production of biooil.