Learn Spanish (Latin America): El mayorista calcula el costo.

Nouns ending in -ista that refer to professions or people do not change their ending based on gender. You use el mayorista for a male wholesaler and la mayorista for a female one. Because it says el mayorista, we know it refers to a male or is being used generally.

Is there a difference between el costo and el coste?

Both words mean the exact same thing, but el costo is the standard term used in Latin America. You will mostly hear el coste if you are speaking with someone from Spain.

Could calcula also mean 'is calculating'?

Yes. In Spanish, the simple present tense like calcula can translate to both 'calculates' (a routine or general fact) and 'is calculating' (happening right now). You only need to use the present progressive form (está calculando) if you want to heavily emphasize that the math is being done at this exact moment.

How would I use this lesson's topic to say the wholesaler calculates 'double the cost'?

You would add el doble de right before the noun. Because the word for cost is el costo, the preposition de and the article el combine to form the contraction del. The phrase becomes: El mayorista calcula el doble del costo.