Spam is hated. Spam often uses dollar signs in the subject or body. It is okay if you are offering a price and put $9.99 in the body. Never put it in the subject. Never use more than one $ sign. Your email users have filters in place with their anti-spam tools at the ISP and email level. It means the ISP, email provider, and clients all try to limit the amount of spam that makes it to the inbox. Dollar signs are a sure way to wind up in the spam and trash folders, thus avoid using $ whenever possible.