SR4 or SR5?
If the latter, you're SOOL.
If the former, however, you can look up Running Wild. Hellhounds are specifically mentioned in one of the examples.
A trained Security Hellhound has a base cost of 10,000 nuyen, multiplied by 10 (Combat Training/Against Instincts), and then multiplied by 2 (Willpower 3) for a total price of 200,000 nuyen
So yeah, not cheap.
But, you can get a ferret pretty inexpensively! :p
Also note that all trained animals come with a minimum lifestyle rating necessary to keep them, and you need the animal handling/training skills to actually train them unless you pay someone else to do it. It's all detailed in the book.