Learn Finnish: Tämä on normaali hinta.

You attach the suffix to the inflectional stem of the adjective. For normaali, the stem is just normaali-. Since it contains back vowels (o, a), you use the back vowel harmony variant -hko, making it normaalihko. The sentence becomes Tämä on normaalihko hinta.

Could I attach -hko directly to the noun hinta to mean 'somewhat of a price'?

No, the -hko / -hkö suffix is only added to adjectives and a few adverbs, never nouns. If you wanted to talk about the price being 'somewhat high', you could use the adjective hintava (pricey) and add the suffix to get hintavahko (somewhat pricey).

Even though this is B2, I sometimes doubt myself: why is it normaali hinta and not the partitive normaalia hintaa?

This is a classic 'A is B' (predicative) sentence. Because the subject tämä refers to a singular, countable thing (a specific price), the complement must be in the nominative case. You would only use the partitive if the subject was an uncountable mass noun (like Tämä on normaalia kahvia) or an abstract concept.

Normaali is obviously a loanword. What native Finnish adjective could I use here, and how does it take the -hko suffix?