La franquicia es de cien euros.

Not usually in this sentence. In Spain, franquicia very often means the deductible / excess on an insurance policy or car rental.

So La franquicia es de cien euros would typically mean that the deductible/excess is 100 euros.

The word franquicia can also mean franchise in the business sense, but the context here strongly suggests the insurance/car-rental meaning.

Because franquicia is a feminine noun in Spanish, so it takes the feminine article la.

• la franquicia
• una franquicia

This is something you mostly just learn with the noun. The ending -a is a strong clue that it is feminine, though there are exceptions in Spanish in general.

In this sentence, ser de + amount is a common way to express the amount/value/size of something.

So:

• La franquicia es de cien euros = the deductible amounts to 100 euros / is 100 euros

The de links the thing being described with the amount.

You will see similar patterns such as:

• La multa es de doscientos euros
• La entrada es de quince euros

It is a very natural structure in Spanish.

Because 100 is cien when it comes directly before a noun.

• cien euros
• cien personas
• cien libros

But use ciento in larger numbers:

• ciento uno
• ciento veinte
• ciento cincuenta euros

So the rule is:

• cien = exactly 100 before a noun, or when standing alone
• ciento = 101–199

Because the grammatical subject is La franquicia, which is singular.

• La franquicia = subject
• es = singular verb agreeing with that subject
• de cien euros = complement telling you the amount

So Spanish is not making the verb agree with euros here. It agrees with franquicia.

That is why es is correct.

You may hear similar patterns in speech with prices or amounts, but for this sentence the standard and safest form is:

• La franquicia es de cien euros

That is the clearest choice for learners.

If you changed the structure so that the amount comes first, Spanish can sometimes behave differently in real usage, but you do not need that here. For this exact sentence, es de cien euros is the normal model to follow.

Normally, no. The natural Spanish sentence is:

• La franquicia es de cien euros

Leaving out de sounds unnatural in standard Spanish here.

Compare:

• Cuesta cien euros = It costs 100 euros
• Es de cien euros = It is 100 euros / It amounts to 100 euros

So if you use ser in this kind of amount expression, de is normally part of the structure.

Because after a number, Spanish normally uses the noun directly, without an article.

• cien euros
• dos coches
• tres días

So you do not say:

• cien los euros
• cien unos euros

Just cien euros.

Also, euros is plural because the amount is more than one euro.

Yes. Cien euros is simply the amount written out in words.

So these are equivalent in meaning:

• La franquicia es de cien euros
• La franquicia es de 100 €

In formal writing, either may be used depending on style. In speech, of course, you would say cien euros.

In standard Peninsular Spanish, it is approximately:

• fran-KEE-thya

A more detailed breakdown:

• fran as in frahn
• qui = kee
• cia in Spain usually sounds like thya because c before i is pronounced like the th in think

So in much of Spain:

• franquicia ≈ fran-KEE-thya

In Latin America, that last part is usually more like:

• fran-KEE-sya

Both are normal in their respective varieties.