What's the medium? Is it for online banner, adwords, billboard, magazine?
The most I've paid for a single piece of advertising had to do with sponsorship at a golf tournament, but that targeted a pretty wide audience.
The most I've paid for a single Adword was $30 per click, but the return on investment was huge due to the product being sold. Other industries such as Cars and Real Estate I've paid anywhere from $7-20 CPM vertical banners on sites. Can get pretty expensive when you have a few hundred thousand visits viewing the advertisements, and limited click through.
If I'd never used the place before, that can be tricky. You'll want to review past stats information for your industry - nothing wrong with putting SOME money into a mini campaign, and then if it works, hit it hard, but you'll want to test the waters rather than commit 100% right out of the gate (usually).