Learn French: Ce studio coûte cher, alors nous travaillons dur. - This studio is expensive, so we work hard.

In French, a few specific adjectives act directly as adverbs when paired with certain verbs. Here, cher goes with coûter and dur goes with travailler. When used in these set verb phrases, they drop the need for an -ment ending.

If we were talking about several houses (maisons), would cher become plural and feminine (chères)?

No, it remains exactly as it is: coûtent cher. Because cher is functioning as an adverb modifying the verb coûter, it is invariable. It will always stay in its masculine singular form, no matter what subject is doing the costing.

Can I say travailler difficilement instead of travailler dur?

No, because they mean different things. Travailler difficilement means working with difficulty or struggling to do the work, perhaps because you are sick or the task is confusing. To say you are putting a lot of effort into your job, you must use the set phrase travailler dur.

Could I use donc instead of alors in this sentence?

Yes, both work perfectly here to mean so or therefore. Alors is very common in everyday speech to show a simple consequence, while donc sounds just slightly more formal and emphasizes logical deduction.