Since you appear to be asking about Quote based on the way you've tagged your question. There's a difference between what you put into the system and what you send to your customer. For the sake of calculations, SF would need a quantity of at least 1 unit in order to calculate a price. Otherwise, null x unit price (or bundle price) = null. So, if a quantity of 1 is there in Opportunity, in a custom quote page you could easily hide the quantity when sending the quote to the customer.
This might also be easier for you if you create your products as what many refer to as "bundles", rather than specific products that are similar to "widgets" one produces and keeps an inventory of that's based on units and unit prices. Although clearly there's a correlation between impressions and rates that you're going to use. I've created a number of custom PDF quote pages that have included "bundles" and that's why I'm relating that paradigm to you in this post.
The easiest way to create your own custom quotes is to install the unmanaged package from Salesforce Labs called Simple Quote/Invoice/Order and use the controller that comes with it as the basis for creating your own custom controller (if you need to customize it) along with the PDF template to create your own custom quote page that omits (or simply hides) the quantity field.
You could also modify it without much difficulty to show a rate and impressions calculation by creating a custom product or product bundle class. I've done things along those lines before on quote templates when creating bundles.
