Are you talking about paying for ads like banner ads, etc? Sometimes this can be very cost effective as banner ads are generally quite cheap. Not a great tactic for sales but good for building your brand if you have a catchy banner.

If you're paying someone else to put up advertising for you, then I wouldn't recommend it unless it's a very good service. Many of these services post your ad at a bunch of crappy FFA sites that no one actually looks at.