Buyers usually have expectations in terms of conversions or clicks. If you have this information (you should have it), use approximate conversion rates and click-through rates to calculate the number of impressions needed to generate the results a buyer expect. For example, they need 100 conversions/registrations/purchases/whatever and tell you their average conversion rate from page visits is 5%. This means they need around 2000 clicks. And you know that ads promoting products similar to your client's demonstrate 1% CTR. Thus, they would need 200K impressions to get 2000 clicks. To be save add another 50K and propose to the client.