On the basis of detailed model calculations we demonstrate that the free energy required to form an equilibrated spherical bilayer vesicle is made up of two main contributions : the (size-independent) work of bending the constituent monolayers and the work of stretching the bilayer that is determined by the planar bilayer tension. A major part of the bending free energy arises due to a reduction of the overall hydrocarbon chain volume upon forming a spherically shaped bilayer shell out of a planar monolayer, while keeping the total hydrocarbon/water contact area fixed and the chemical potentials constant. This contribution is only partially counterbalanced by a sum of contributions due to curvature-dependent molecular free energies of various origins. The overall free energy of bending a pure (charged) SDS bilayer into a vesicle is found to be quite large (similar to 90kT). However, admiring with a long-chain alcohol brings it down substantially and is thus crucial for spontaneous vesicle formation to occur.