Ma koliko je rezervni deo skup, procena troškova je potrebna.

Ma koliko is a concessive expression meaning no matter how, however, or regardless of how.

So Ma koliko je rezervni deo skup... means something like No matter how expensive the spare part is...

It introduces a contrast:

• even if something is true,
• the main clause still remains true.

In this sentence, the idea is:

Even if / no matter how expensive the spare part is, a cost estimate is still necessary.

This is because je is a clitic in Serbian. Clitics are short unstressed words that usually appear in the second position of their clause.

In the clause:

Ma koliko je rezervni deo skup

the first unit is Ma koliko, so je comes right after it.

This second-position behavior is very common in Serbian:

• On je ovde.
• Kad je došao...
• Ako se sećaš...

So the placement of je is about Serbian clitic word order, not about emphasis.

Because skup agrees with rezervni deo, which is a masculine singular noun.

Agreement in Serbian means the adjective changes form to match the noun in:

• gender
• number
• case

Here:

• deo = masculine singular
• therefore skup = masculine singular

Compare:

• skup deo = an expensive part
• skupa knjiga = an expensive book
• skupo gorivo = expensive fuel

So skup is the correct form because it matches rezervni deo.

Rezervni deo means spare part or replacement part.

It is made of:

• rezervni = spare / replacement
• deo = part

So literally it is spare part.

A learner may also notice that Serbian often uses adjective + noun combinations where English may use a noun + noun structure. English says spare part; Serbian says rezervni deo.

Because troškova is in the genitive plural.

The noun procena means assessment / estimate, and it commonly takes a genitive phrase meaning of something:

• procena troškova = estimate of costs
• procena vrednosti = estimate of value
• procena štete = estimate of damage

So:

• troškovi = costs
• troškova = of costs

This is a very common Serbian pattern: a noun followed by another noun in the genitive to show a relationship that English often expresses with of.

Because it agrees with procena, which is a feminine singular noun.

In the main clause:

procena troškova je potrebna

the adjective potrebna describes procena.

So the agreement is:

• procena = feminine singular
• potrebna = feminine singular

Compare:

• Plan je potreban. = The plan is necessary.
• Procena je potrebna. = The estimate is necessary.
• Rešenje je potrebno. = The solution is necessary.

So the adjective changes form depending on the noun it refers to.

It is understandable, but many speakers would find a version with bio or da je a bit more natural after ma koliko.

Common alternatives are:

• Ma koliko rezervni deo bio skup, procena troškova je potrebna.
• Ma koliko da je rezervni deo skup, procena troškova je potrebna.

These sound a little more idiomatic to many speakers in a formal or careful style.

Your original sentence is still understandable, but a learner should know that ma koliko often appears in patterns like:

• ma koliko bio...
• ma koliko da je...

Yes. Koliko god is often used with a very similar meaning: no matter how / however.

For example:

• Koliko god je rezervni deo skup, procena troškova je potrebna.
• more naturally for many speakers: Koliko god rezervni deo bio skup, procena troškova je potrebna.

Both ma koliko and koliko god can introduce this kind of concessive idea.
Ma koliko may sound a bit more formal or literary in some contexts, while koliko god is also very common.

Both forms exist, depending on the pronunciation standard/dialect:

• deo = Ekavian
• dio = Ijekavian

So:

• rezervni deo and
• rezervni dio

both mean spare part.

This is not a grammar difference in meaning; it is mostly a standard/dialect difference. Serbian in Serbia is often written in Ekavian, so deo is very common there.