Learn Russian: Новые коньки стоят дорого.

Because of the verb. In English, we say skates are expensive (using the verb 'to be' + adjective). In Russian, the verb here is стоят (cost). Since it is an action verb, you need the adverb дорого (literally 'dearly') to describe how they cost.

Wait, doesn't стоят mean 'stand'? How can I tell when it means 'cost'?

It comes down to word stress and context! сто́ят (with stress on the о) means 'they cost'. стоя́т (with stress on the я) means 'they stand'. In written text without stress marks, you just rely on the context. Since skates cannot 'stand expensive', it must mean they cost a lot.

Is коньки always plural? Can I talk about just one skate?

You can talk about one skate using the singular form конёк. However, just like in English, ice skates come in pairs, so you will almost exclusively see and use the plural form коньки in everyday conversation.