Learn Farsi: اولاً، اجاره‌بهای این خانه‌ی کلنگی خیلی گران است. - Firstly, the rent for this run-down house is very expensive.

avvalan means "firstly" and introduces the first point in a sequence. The double dash at the end is an Arabic diacritic called a tanvin, which is pronounced as "-an" in Farsi. It is commonly added to Arabic loanwords to turn them into adverbs. You will see this a lot with sequence markers, such as sâniyan (secondly) and sâlesan (thirdly).

Why is a run-down house called kolangi?

The word kolangi comes from kolang, which means "pickaxe". In Iranian real estate, a xâne-ye kolangi is a house so old or dilapidated that its only real value is the land it sits on. It is essentially a "tear-down" property that is ready for the pickaxe.

Why do we add the -ye sound to the end of ejâre-bahâ?

ejâre-bahâ is a compound noun meaning "rent" (literally "rent-price"). Because the second part of the word (bahâ) ends in the long vowel â, it requires a consonant y as a bridge before adding the ezâfe vowel e to link it to the rest of the noun phrase. Therefore, it is pronounced ejâre-bahâ-ye.

Is it natural to use gerân (expensive) to describe rent, instead of saying the rent is "high"?

Yes. While English speakers normally say the rent or the price is "high", Farsi speakers comfortably use gerân (expensive) and arzân (cheap) to describe rent, prices, and bills. Saying the rent is bâlâ (high) would also be correct, but is a very natural collocation here.