preti-
“back and forth, value given in return”price, praise, precious, appreciate
Root yielding Latin pretium, English price, praise, precious, appreciate, depreciate.
Discussion
The PIE form *preti- (back, in return, against — with the extended sense of value given in exchange) derives from the adverbial/prepositional root *per- (forward, through) with a suffix marking reciprocal motion. The meaning is "in return" — and from this concept of reciprocity, the IE languages built their vocabulary of economic value.
Latin pretium (price, value, worth — that which is given in return) continues the root and generated one of English's most important commercial and evaluative word families: price (the amount given in return for something), precious (of great price, highly valued), appreciate (ad-pretiāre, to set a price on — later to recognise value), depreciate (dē-pretiāre, to lower the price — to lose value), praise (from Old French preisier, to value, to appraise — to praise someone is etymologically to price them highly), and appraise (to set a price on, to evaluate).
The connection between praising and pricing deserves emphasis: to praise and to appraise share the same root because both are acts of evaluation. The market price and the verbal praise are both judgments of worth — the merchant prices goods as the critic praises art. This equation of value with vocal judgment is preserved in the very sound of the words.
Sanskrit práti (against, in return, toward) preserves the adverbial/prepositional sense without the specific economic extension. The Vedic práti appears in compounds expressing reciprocity and opposition.
Old Church Slavonic protivu (against) and Lithuanian priešas (enemy — one who stands against) show the adversative sense: what is "in return" can be either commercial exchange or hostile opposition.
The root's semantic journey from spatial "facing/returning" to economic "pricing" to evaluative "praising" encodes the conceptual origin of value itself: worth is what comes back to you, what is returned in exchange, what faces you in the transaction. The PIE speakers who said *preti for "in return" laid the foundation for three millennia of economic thought.