It's a matter of something being singular or plural. Example: If a car lot is focusing an advertisement on their many vehicles, they would say "we have affordable prices" to indicate that no matter which of the many vehicles you are shopping for, the prices (in general) are affordable. If a car lot is focusing an advertisement on just one vehicle, they would say they have it now "at an affordable price" to indicate that the one vehicle is affordable.

Typically you would hear "at an affordable price" unless you're talking about prices being in the plural form.