Unit price is the price per item, but that's a term usually used when referring to tangible products. For example, if I buy a package of 8 bottles of shampoo for 240 euros, than my unit price is 30 euros per bottle.
As explained in Wikipedia, in the realm of business, unit cost is a business term that describes how much each unit costs to make, including materials, assembly, and labor.
I think you might be better off using web advertising terminology. Cost per display and cost per click are more common units of pricing for banner ads. Because each display is usually very inexpensive, displays are often measured in thousands:
CPC - Cost per click (CPC) is one of the online payment models by which advertisers pays for each click through made on their advertisement. This is an ideal method of payment for advertisers who need to guarantee they only pay for those viewers of the banner that click on it and visit a page on their site.
CPM - Cost per thousand (CPM) is one of the online payment models by which advertisers pays for every 1000 impressions of their advertisement. Prices typically range from $10 to over $50 per thousand impressions. This is an ideal method of payment for advertisers who want to guarantee only the number of people who sees their banner. The "M" in CPM is from the Roman numeral for 1000. The Roman numeral M was derived from the Latin word "mille" meaning "thousand".
Source: Web Advertising Glossary. A similar glossary can be found here.
Some websites (like this one) mention that CPC is a more popular model than CPM, but that issue is outside the scope of an English forum.