Learn Finnish: Kana on halvempaa kuin lohi. - Chicken is cheaper than salmon.

Because we are talking about chicken as a food or substance (an uncountable mass noun), not as one single, countable bird. When describing an uncountable subject, the adjective describing it must be in the partitive case, even if the subject itself is nominative.

How do we build the word halvempaa from the basic adjective halpa?

First, we form the comparative (cheaper) by adding the ending -empi, giving us halvempi. Notice that the consonant p softens to v here. Then, to make it partitive, the final -i becomes -aa, resulting in halvempaa.

If chicken is an uncountable mass here, why is the subject kana in the nominative case instead of the partitive kanaa?

In Finnish 'A is B' sentences, the subject usually stays in the nominative case when making a general statement about what that thing is like. It is the descriptive adjective (halvempaa) that takes the partitive case to show that the subject is an uncountable substance.

Is there another way to say 'than salmon' without using the word kuin?