Origins
To praise is to put a price on someone's worth. The word comes from Old French preisier — 'to prize, to value' — from Late Latin pretiāre, 'to value', from Latin pretium, 'price, value, reward'.
The economics of admiration run deep. Praise, price, prize, precious, appraise, appreciate, and depreciate all descend from the same Latin root. In the medieval mind, there was no gap between declaring something valuable and declaring it admirable. To praise a knight was to assess his worth. To prize a jewel was the same act applied to a stone.
English eventually split these meanings into separate words. Price kept the commercial sense. Prize became the reward. Precious meant 'of great price'. Appraise stayed close to the original: to assess value formally. Praise drifted toward purely emotional and spiritual territory — especially once the Church adopted it for worship. To praise God was the ultimate act of valuation: declaring infinite worth.
Later History
French never made such a clean split. The verb priser still means both 'to prize' and 'to value'. The noun prix means both 'price' and 'prize' (the Prix in Grand Prix is the same word).
The Proto-Indo-European root *pret- meant 'to sell, to exchange'. Commerce and admiration grew from the same seed: both ask the same question — what is this worth to you?