The Spanish verb 'costar' (to cost) comes from the Latin word 'constare', which meant 'to agree' or 'be consistent'. The Latin 'constare' itself was formed by combining two elements: the prefix 'con-' meaning 'together' or 'with', and the verb 'stare' meaning 'to stand'. So literally, 'constare' meant 'to stand together', which evolved to mean something that stands firm or is agreed upon, like a price or cost.

Some related Spanish words include 'constar' (to be evident or clear), which maintains a meaning closer to the original Latin sense of being established or consistent. Another related word is 'estar' (to be), which comes from the same Latin root 'stare' (to stand).

English speakers might recognize this etymology in words like 'constant' and 'consistent', which come from the same Latin root 'constare'. Just as these English words convey the idea of something that stands firm or remains unchanged, the Spanish 'costar' refers to the established or standing price of something. The English word 'cost' itself is also related, having evolved from the same Latin source.