En el examen, me cuesta menos sumar que dividir.

Because costar is a very common Spanish verb for saying that something is difficult or takes effort.

In this kind of structure, me cuesta means something like:

• it is hard for me
• it takes effort for me
• I find it difficult

So me cuesta menos sumar que dividir means that adding requires less effort for you than dividing.

Me is an object pronoun meaning to me or for me here.

Spanish does not usually say this idea as I am difficulting or something similar. Instead, it treats the activity as the thing that is difficult, and the person is marked with an indirect object pronoun:

• Me cuesta sumar = adding is hard for me
• Te cuesta sumar = adding is hard for you
• Le cuesta sumar = adding is hard for him/her

So me is not reflexive here. It does not mean myself. It means to me / for me.

Because the subject is the action sumar, which is an infinitive, and infinitives are treated as singular ideas.

So:

• Sumar me cuesta menos → sumar is one action, so cuesta
• Las divisiones me cuestan menos → las divisiones is plural, so cuestan

Even though both sumar and dividir appear in the sentence, the comparison is still built around singular action words.

Because Spanish uses the infinitive to talk about actions in a general way.

Here, sumar and dividir mean:

• adding
• dividing

This is one of the places where English often uses -ing forms, but Spanish uses the infinitive instead.

So:

• Me gusta leer = I like reading
• Me cuesta estudiar = studying is hard for me
• Me cuesta menos sumar que dividir = adding is less hard for me than dividing

Menos ... que is the normal pattern for saying less ... than.

In this sentence, it compares difficulty or effort:

• me cuesta menos sumar que dividir

That means adding is less difficult than dividing.

A helpful way to see it is:

• menos = less
• que = than

So the pattern is:

• [something] cuesta menos [action 1] que [action 2]

Yes. This is a case of omission of repeated words, which is very common.

The full idea is something like:

• Me cuesta menos sumar que dividir
• understood as: Me cuesta menos sumar que dividir me cuesta or, more naturally in English, Adding is less hard for me than dividing is

Spanish normally leaves out the repeated part because it is obvious from the context.

It sets the context: in the exam or on the exam.

In natural English, you might say on the test or in the exam, depending on the situation. In Spanish, en el examen is a very normal way to mean during the exam / in the context of the exam.

It tells us that this comparison is specifically about exam conditions, not necessarily about maths in general.

El examen usually refers to a specific exam or a known exam situation.

So it sounds like the speaker has a particular exam in mind:

• En el examen... = in the exam / on the test

If you said en un examen, it would sound more general:

• En un examen, me cuesta menos sumar que dividir = in an exam / on a test, I find adding easier than dividing

Both are possible, but el examen sounds more specific.

Yes. Spanish word order is flexible.

The original sentence is very natural:

• En el examen, me cuesta menos sumar que dividir.

But you could also say:

• Me cuesta menos sumar que dividir en el examen.
• Sumar me cuesta menos que dividir en el examen.
• En el examen, sumar me cuesta menos que dividir.

The original version is especially natural because En el examen gives the context first.

Yes. A few common alternatives are:

• En el examen, me resulta más fácil sumar que dividir.
• En el examen, se me da mejor sumar que dividir.

These are similar, but not identical:

• me cuesta menos = it takes less effort for me
• me resulta más fácil = I find it easier
• se me da mejor = I’m better at it

Your original sentence is completely natural and standard.

No. Although costar can mean to cost in the money sense, it also very often means to be difficult or to take effort.

For example:

• Cuesta diez euros = It costs ten euros
• Me cuesta entenderlo = It’s hard for me to understand it

So in your sentence, costar is about difficulty, not price.