Learn Hardcore Portuguese (Portugal): Vou comparando os preços no mercado para poupar dinheiro. - I keep comparing the prices at the market to save money.

Estou a comparar os preços = I’m comparing the prices (right now, at this moment).
Focus: one concrete, current action.
Vou comparando os preços = I keep comparing / I go on comparing prices (over time).
Focus: a process that happens gradually or repeatedly, often as you go along: every time you shop, each time you see a product, etc.

So vou comparando suggests a continuous or repeated habit rather than a single, current activity.

Is vou comparando a future tense, like “I’m going to compare”?

No. The future meaning appears when ir is followed by an infinitive, not by a gerund.

Vou comparar os preços. = I’m going to compare the prices (in the future / later).
Vou comparando os preços. = I (keep) comparing prices / I go on comparing prices (over time).

So:

ir + infinitive → near future: going to do something
ir + gerúndio (ir comparando, ir fazendo, ir vendo…) → an action that develops gradually or is repeated over a period.

I learned that European Portuguese doesn’t really use the gerund (-ando, -endo). Why do we have comparando here?

What you learned is only partly true.

In European Portuguese:

For the progressive (“I am doing”), people normally use
estar a + infinitive:
- Estou a comparar os preços.
- (Rather than estou comparando os preços, which is Brazilian-style.)
But the gerund is still used in some very common structures, especially with verbs like ir, vir, ficar, passar:
- Vou comparando os preços.
- Vai chovendo. (It keeps raining / it’s slowly raining on and off.)
- Foi ficando nervoso. (He gradually became nervous.)

So vou comparando is perfectly natural in European Portuguese and expresses a gradual, ongoing process. It’s not the same construction as the Brazilian-style estar comparando.

Could I just say Vou comparar os preços no mercado instead? What’s the difference?

You can, but the nuance changes:

Vou comparar os preços no mercado.
= I’m going to compare the prices in the market (at some point / later).
→ A future plan or intention, a more defined one-time action.
Vou comparando os preços no mercado.
= I keep comparing prices in the market.
→ A repeated or ongoing habit, done gradually (for example, every time you go shopping).

So it’s not a grammar mistake to use vou comparar; it just loses the idea of gradual, repeated action that vou comparando has.

Why is it os preços and not just preços? Are both correct?

Both are possible, but they feel slightly different.

Vou comparando os preços no mercado…
The article os makes it sound like a specific set of prices in a given context (e.g., the prices of the products you’re looking at in that market).
Vou comparando preços no mercado…
Without the article, it sounds a bit more general: the speaker compares prices in general, not a clearly delimited set.

In many everyday contexts, both would be understood the same way. Using os preços just highlights that we are talking about those particular prices in that market.

What exactly does no mercado mean here? A physical market, or “the market” in an economic sense?

On its own, no mercado most naturally means “in the (physical) market”, like a street market, local market, or sometimes a supermarket-type space, depending on context.

Vou comparando os preços no mercado…
→ Usually: I compare prices at the market (where I shop).

If you want the more abstract, economic sense (in the housing market, in the job market), Portuguese usually specifies which market:

no mercado imobiliário – in the housing/real estate market
no mercado de trabalho – in the job market

So in your sentence, it most likely refers to a physical place where you buy things.

Why is it no mercado and not em o mercado?

No is simply the contraction of the preposition em + the definite article o:

em + o = no
em + a = na
em + os = nos
em + as = nas

So:

no mercado = in the market
literally em o mercado, but in correct Portuguese it must contract to no.

You virtually never say em o mercado; the contracted form is obligatory.

Why do we use para in para poupar dinheiro, and not por?

Because para is the normal preposition to express purpose / goal:

para + infinitive = in order to / to (do something)

So:

Vou comparando os preços no mercado para poupar dinheiro.
= I (keep) comparing prices in the market *to save money / in order to save money.*

Using por would give a different idea (cause, reason, duration, etc.) and would be wrong here:

por poupar dinheiro is not how you say in order to save money.

So for purpose, stick with para + infinitive:
para poupar, para aprender, para descansar, etc.

Could I use another verb instead of poupar for “save money,” like guardar or salvar?

In this context, poupar is the natural choice.

poupar dinheiro = to save money by spending less / cutting costs.
This is exactly what comparing prices is about.

Other verbs:

guardar dinheiro
Literally “to keep money / put money aside”.
More about not touching the money or putting it away (e.g., in a drawer or bank), not necessarily about finding cheaper prices.
salvar dinheiro
Very unusual in Portuguese for this meaning. Salvar is mostly “to save” in the sense of rescue (a person, a file, a situation), not money.
economizar dinheiro
Also means “to save money” and can be used, but in European Portuguese poupar dinheiro is more common and natural in everyday speech.

So here, poupar dinheiro fits best with the idea of comparing prices to spend less.