Impossible to say with the limited information provided.
A few questions:
How well do the visitors to the site match with your target demographic? If you sell organic baby food, you'd do a lot better advertising on a site catering to new moms than on one targeting hard-core gamers, for instance.
What sort of profit do you make per sale? (Profit, not gross revenue.) If you make $1 in profit per sale, this site would need to refer you at least 25 sales (not visitors, actual sales) per month for you to justify the ad spend. On the other hand, if you make $100 per sale, one sale every four months would get you to break-even.
What sort of commitment do you need to make? Is it month-to-month, so you can back out at any time if it turns out the ad spend isn't generating good ROI? Or are you going to be locked in to a longer-term contract, where you could find yourself throwing away $25 a month on a non-productive ad?
How much control do you have over the ad creative? What sort of ad is it (text, banner, graphic, etc)? What sort of ad placement/rotation will you get? Who among your competitors (if any) is already advertising there?
Ponder these questions, and I suspect the answer will become clear to you.