YoVille will NEVER ask for your password, so please be wary of malicious emails and/or forum posts that request your password, including via web links.
There is no reliable guide. But I do know that is one tough pet to find. The last time (2 years ago?) I saw anyone posting in events, I recall them asking 35M for it. I do not know if it sold.
I was seeing Cleo going for around 5m last time I saw. Maybe you could do an event saying you're paying 5m or so, and see if that works before you go higher.
If you're selling, I guess you could do the opposite. lol