It depends on several factors: your website niche and traffic sources, seasonalities, the ad network's rates, and how you place your ads. Realistically speaking, you can expect to earn $1–10 per 1,000 pageviews. If your traffic is low, your ad revenue will be small but it will grow as your traffic numbers ramp up. In general, display ads will not make you rich overnight with low traffic, but it can provide a steady passive income stream that grows with your site.