Most ezines offer discount packages for bulk advertising. The advantage of giving an email address is that it gives you the opportunity to send a powerful sales letter to the person who responded to your ad. Offer something free in your ad copy. It'll often tip the balance between a response and no response. Keep your ads short, even if you're not using the number of words you're allowed.
Short ads are more likely to be read.
Use the word 'You'. Don't describe your product but tell the reader what your product can do for them.