Choose the appropriate preposition for the given sentence:
Our prices are subject ______ a heavy discount.

There are some verbs/nouns/adjectives which are followed by fixed prepositions as given below: ​
Exonerate from, refrain from, subject to, based on, aptitude for, inured to, accused of, predilection for, respite from, vexed at, etc.
Example: ​After spending some time on the island, they became inured to the hardships.​ ​
Hence, according to the phrases given above, 'to' will be the correct choice.
The given phrase 'subject to' means cause or forced to undergo (a particular experience of a form of treatment).
Correct Sentence: Our prices are subject to a heavy discount.