Harga itu cukup rendah.

In Indonesian, a present-tense to be verb is often omitted. So a noun phrase can be followed directly by an adjective.

• Harga itu rendah = That price is low
• Harga itu cukup rendah = That price is quite low

So even without a separate word for is, the sentence is complete and natural.

Itu usually means that, but after a noun it can also help make it definite, somewhat like the in English depending on context.

So harga itu can mean:

• that price
• the price

Which one sounds best depends on the situation.

Here cukup means quite, fairly, or rather.

• rendah = low
• cukup rendah = quite/fairly low

In other contexts, cukup can also mean enough, so it is a flexible word.

For example:

• Uangnya cukup. = The money is enough.
• Harga itu cukup rendah. = The price is quite low.

Rendah means low on a scale, while murah means cheap/inexpensive.

So:

• harga rendah = low price
• barang murah = cheap item
• hotel murah = cheap hotel

With harga, rendah is very logical because a price can be high or low. In everyday Indonesian, people do also say harga murah, but harga rendah is a bit more literally low price.

The structure is:

• Harga itu = subject/topic = that/the price
• cukup rendah = predicate = quite low

So the sentence follows a common Indonesian pattern:

subject + adverb + adjective

Compare:

• Harga itu rendah. = The price is low.
• Harga itu cukup rendah. = The price is quite low.

Yes, you often can:

• Harga cukup rendah.

This sounds more general or less specifically tied to one already-identified price. Adding itu makes it more definite or points to a specific price already being discussed.

So both are possible, but they feel slightly different in context.

Yes. Harganya cukup rendah is also very natural.

Harganya literally means something like:

• its price
• the price

In many situations, harganya is a very common way to say the price. Compared with harga itu:

• harga itu = that price / the price
• harganya = its price / the price

Both are correct, but harganya often sounds especially natural in conversation.

Yes.

• Harga ini cukup rendah. = This price is quite low
• Harga itu cukup rendah. = That price is quite low

The difference is simply:

• ini = this
• itu = that

Both normally come after the noun.

Yes, rendah is an adjective.

In this sentence, though, it is acting as the predicate of the whole sentence, not just as a modifier inside the noun phrase.

• harga rendah = low price
  Here rendah directly modifies harga.
• Harga itu rendah. = That price is low.
  Here rendah is the predicate.

More generally, Indonesian adjectives usually come after the noun they describe:

• rumah besar = big house
• mobil baru = new car

A simple learner-friendly pronunciation is:

HAR-ga EE-too CHOO-koop ren-DAH

By syllable:

• har-ga
• i-tu
• cu-kup
• ren-dah

A few useful pronunciation notes:

• c sounds like ch in chair
• u sounds like oo in food
• h is pronounced
• r is usually tapped or lightly rolled