Jo flere kubikkmeter vi leier, jo høyere er månedsleien.

It is the standard Norwegian way to form a correlative comparative pattern, the same basic idea as English the more ..., the ...er ....

So:

• Jo flere kubikkmeter vi leier, jo høyere er månedsleien.

works like:

• The more cubic meters we rent, the higher the monthly rent is.

A useful pattern to remember is:

• Jo + comparative + clause, jo + comparative + clause

Examples:

• Jo mer du øver, jo bedre blir du.
  = The more you practice, the better you get.

• Jo tidligere vi drar, jo mindre trafikk blir det.
  = The earlier we leave, the less traffic there will be.

Here, jo does not mean yes. It is a different use of the word.

Because kubikkmeter is a countable noun here.

In Norwegian:

• flere = more, with countable things
• mer = more, with uncountable things or amounts in general

So:

• flere kubikkmeter = more cubic meters
• mer vann = more water
• mer plass = more space

Since you can count one cubic meter, two cubic meters, three cubic meters, Norwegian uses flere.

Many measurement words in Norwegian are often used with no visible plural ending, especially in practical or technical contexts.

So it is normal to say:

• én kubikkmeter
• to kubikkmeter
• flere kubikkmeter

This is similar to some other unit words that can stay the same in form in everyday usage.

So even though the meaning is plural, the form kubikkmeter is completely normal here.

Høyere is the comparative form of høy.

• høy = high
• høyere = higher
• høyest = highest

Because the sentence compares one level of rent with another, Norwegian needs the comparative:

• jo høyere = the higher

This is exactly the kind of form used after jo in this pattern.

Because Norwegian often uses the definite form when talking about a specific thing in a general relationship like this.

• månedsleie = monthly rent
• månedsleien = the monthly rent

In this sentence, we are talking about the monthly rent for this arrangement, so the definite form sounds natural.

This is very common in Norwegian, where English might sometimes sound more natural without the, but Norwegian prefers the definite form.

Because Norwegian normally writes compound nouns as one word.

So:

• måned
  • leie → månedsleie

This is one of the most important spelling differences from English. English often writes noun combinations as two words, but Norwegian usually combines them.

Other examples:

• bokhylle = bookcase
• skolebuss = school bus
• sommerferie = summer holiday

The -s- in månedsleie is a linking sound that often appears in compounds.

Because Norwegian follows its usual verb-second word order quite strongly in main-clause-like structures.

After the fronted phrase jo høyere, the verb comes next:

• jo høyere er månedsleien

This may feel unusual to an English speaker, because English would normally say:

• the higher the monthly rent is

Norwegian often puts the verb before the subject after an element has been moved to the front.

Compare:

• I dag er han hjemme.
• Nå kommer bussen.

So in your sentence:

• jo høyere = fronted comparative phrase
• er = verb
• månedsleien = subject

That word order is normal.

Leier is the present tense of å leie, which can mean to rent / to hire / to lease, depending on context.

Here it means to rent.

So:

• vi leier = we rent / we are renting

Be aware that å leie can be used in different situations:

• leie en bil = rent a car
• leie en leilighet = rent an apartment
• leie lagerplass = rent storage space

So the meaning depends on what is being rented.

No. Å leie means to rent / hire, not to borrow.

Useful distinctions:

• å leie = to rent
• å låne = to borrow / lend, depending on structure

Examples:

• Jeg leier en leilighet.
  = I rent an apartment.

• Jeg låner en bok av henne.
  = I borrow a book from her.

So in your sentence, the idea is about paying for rented space, not borrowing something for free.

The comma separates the two parts of the jo ... jo ... structure.

• Jo flere kubikkmeter vi leier, jo høyere er månedsleien.

This makes the sentence easier to read, because the first part sets up the comparison, and the second part gives the result.

In practice, you will very often see a comma in sentences like this.

Yes, that is a very good way to think about it.

A very reliable translation pattern is:

• Jo flere ..., jo høyere ...
• The more ..., the higher ...

More examples:

• Jo eldre han blir, jo roligere blir han.
  = The older he gets, the calmer he becomes.

• Jo mer jeg leser, jo mer forstår jeg.
  = The more I read, the more I understand.

So if you see jo ... jo ..., your first instinct can usually be: the more / the less / the better / the higher ... the ...

Er is perfectly natural here because it simply states a condition:

• månedsleien er høyere = the monthly rent is higher

But in other similar sentences, Norwegian often uses blir when the idea is more dynamic or result-based:

• Jo mer du trener, jo sterkere blir du.
  = The more you train, the stronger you become.

In your sentence, er sounds natural because it describes the rent level as a fact tied to the amount rented.

Yes. The jo ... jo ... pattern is very standard and natural, but you could also express the same idea more directly.

For example:

• Månedsleien blir høyere hvis vi leier flere kubikkmeter.
  = The monthly rent becomes higher if we rent more cubic meters.

That said, the original sentence is not especially formal. It is just a very common comparative structure in Norwegian, and it is worth learning well because it appears often.