Per annum, per centum, per mille, per capita, et cetera, et cetera.

These are Latin phrases, borrowed especially in British English as Great Britain was occupied by Latin speakers for nearly 400 years -- 43 CE through 410 CE. Latin continued to be the language of diplomacy, religion, philosophy, and science through the 18th and 19th century.

It is, for all practical intents, proper British English.

https://en.m.wiktionary.org/wiki/per

Though yes, $<value>/yr. is more frequently seen especially in American English.