Cada unidade custa cinco euros.

Because cada is followed by a singular noun in Portuguese.

So you say:

• cada unidade
• cada pessoa
• cada livro

Even if you are talking about many individual items, Portuguese still uses the singular after cada.

So cada unidade custa cinco euros is correct, while cada unidades is not.

Cada means each or every, depending on the context.

In this sentence, cada unidade is best understood as each unit.

A useful point:

• cada does not change for gender or number
• it stays cada in all cases

Examples:

• cada dia = each day
• cada casa = each house
• cada criança = each child

The subject is cada unidade, which is grammatically singular.

Because of that, the verb must also be singular:

• cada unidade custa
• not cada unidade custam

This is the same idea as in English:

• Each unit costs... not
• Each unit cost...

The verb custa is the 3rd person singular of custar in the present tense.

The infinitive is custar, which usually means to cost.

In price sentences, it works very much like English:

• Isto custa cinco euros. = This costs five euros.
• Quanto custa? = How much does it cost?
• Cada unidade custa cinco euros. = Each unit costs five euros.

It can also have other meanings in other contexts, such as to be difficult:

• Custa-me acordar cedo. = It is hard for me to wake up early.

But in your sentence, it clearly means to cost.

Because after numbers greater than one, the noun is normally plural.

So:

• um euro
• dois euros
• cinco euros

That is the same basic idea as English:

• one euro
• five euros

If the amount were exactly one, you would say:

• Cada unidade custa um euro.

No. After custar, the price comes directly.

So Portuguese says:

• custa cinco euros
• not custa de cinco euros

This is an important pattern to remember:

• O livro custa dez euros.
• A entrada custa vinte euros.
• Cada unidade custa cinco euros.

Yes, if the context already makes clear what the units are.

Cada uma means each one, and it is feminine because unidade is feminine.

So:

• Cada unidade custa cinco euros. = explicit
• Cada uma custa cinco euros. = more natural when the noun is already understood

If the noun were masculine, you would say:

• Cada um custa cinco euros.

Yes, but it depends on the situation.

Unidade is common in formal, commercial, technical, and packaging contexts, where it means unit or item.

For example:

• vendido à unidade = sold individually / by the unit
• preço por unidade = price per unit

In very everyday speech, people might also say other things depending on what is being sold:

• cada peça = each piece
• cada artigo = each item
• cada um / cada uma = each one

So cada unidade custa cinco euros sounds perfectly natural, especially in pricing or sales language.

A careful European Portuguese pronunciation would be roughly:

Cada unidade custa cinco euros.
KAH-dɐ oo-nee-DAH-d(ɨ) KOOSH-tɐ SING-koo ÊU-roosh

A few useful pronunciation notes:

• cada: the final a is reduced in European Portuguese
• unidade: the final e is often very weak
• custa: the s here sounds like sh, so it sounds close to kooshta
• cinco: the first syllable sounds like sing
• euros: in European Portuguese, the final s sounds like sh in this position

Pronunciation varies a bit by region, but this will help you sound more European Portuguese than Brazilian Portuguese.

You usually keep the normal order:

Cada unidade custa cinco euros.

That is the most standard and natural way.

Other orders may appear in special contexts, but they are less neutral. For example:

• Custa cinco euros cada unidade.

This can be understood, but it sounds more marked and is less likely to be the basic form a learner should use first.

So for normal everyday Portuguese, stick with:

• subject + verb + price
• Cada unidade custa cinco euros.

They are related, but not the same.

• cada unidade = each individual unit
• todas as unidades = all the units

Compare:

• Cada unidade custa cinco euros.
  This focuses on the price of one unit at a time.

• Todas as unidades custam cinco euros.
  This means all the units cost five euros, and now the subject is plural, so the verb becomes custam.

This is a very useful contrast:

• cada + singular noun + singular verb
• todas as + plural noun + plural verb

Yes, absolutely. It sounds natural in a commercial context.

You might see or hear things like:

• Cada unidade custa cinco euros.
• Preço por unidade: 5 euros.
• A unidade custa cinco euros.
• São cinco euros por unidade.

All of these are possible, but your sentence is clear, correct, and natural for Portugal. It sounds especially appropriate in a price list, advert, catalogue, or sales explanation.