Echoing the price differential that Spaceman mentioned, though also thought I'd chime in with the suggestion of stacking both Tirz and Sem for the best bang for your buck protocol.
I've forgotten the pharmacokinetic specifics, but Sema doesn't hit as many pathways, but does have the highest bonding efficacy for the appetite suppression ligands. Hence most potent suppression, but also highest nausea and side effects. Conversely Ret and Tirz hit other pathways which similarly curb appetite in different ways, but also hit receptors involved in anti-nausea. So in combining Sema with a low Tirz dose you get great suppression, with reduced typical Sem sides.