There is no official value guide for pets. Pet values can vary a lot depending on the person and the pet due to a lot of different factors, such as demand, rarity, popularity, etc. Some people prefer to trade strictly on rarity value and ignore any demand value of a pet, some prefer to trade including some or all of demand value, so the answer to your question can vary widely just depending on whether you want to trade strictly rarity-only or include demand.
There are a number of helpful topics on the
CS Discussion board. There are a couple pet value guides on there depending on how you prefer to trade. There are also threads for asking whether a trade is fair, or how much a pet is worth in C$, etc. I'd suggest using those.