Learn Hardcore Japanese: kono resutoran ha ninki ga arimasu. sore ni, nedan mo yasui desu. - This restaurant is popular. Besides, the prices are cheap.

人気があります literally means “there is popularity,” i.e., “it has popularity.” It feels a bit more objective and is a very common set phrase.
人気です treats 人気 as a predicate noun: “(it) is popular.” It’s a little more direct and adjective-like.

You can safely use either:

このレストランは人気があります。
このレストランは人気です。

When modifying a noun, prefer:

人気のあるレストラン / 人気のレストラン (You will rarely see 人気なレストラン; stick to 人気の/人気のある.)

Why do we have both は and が in このレストランは人気があります?

は marks the topic: “As for this restaurant…”
Inside the predicate, 人気がある is a fixed pattern where が marks the thing that exists (popularity): “popularity exists.”

So the structure is “As for this restaurant, (it) has popularity.”

Why あります and not います?

ある/あります is used for inanimate things and abstract concepts. 人気 (popularity) is abstract, so 人気があります.
いる/います is for animate beings (people/animals).

What does それに mean here? How is it different from そして, しかも, それで, また, さらに?

それに: “in addition; besides.” Adds another point of the same orientation.
そして: neutral “and,” often just sequences facts.
しかも: “what’s more,” stronger emphasis/surprise on the add-on.
それで: “so/therefore,” indicates a consequence (not just addition).
また: “also; additionally,” a bit formal/neutral in writing.
さらに: “furthermore; even more,” suggests escalation/intensification.

In your sentence, それに is perfect because you’re adding another positive point.

Is the comma after それに necessary? And what about the spaces between words?

The comma after それに is optional but common: それに、 helps the reader see the connection clearly.
The spaces you see (この　レストラン　は…) are for learners. Standard Japanese writing does not put spaces between words. You would normally write: このレストランは人気があります。それに、値段も安いです。

What exactly is も doing in 値段も安いです?

も means “also/too,” signaling addition to the previous statement:

First fact: It’s popular.
Additional fact: The price is cheap too.

Compare:

値段が安いです = neutral/new information “the price is cheap.”
値段は安いです = sets “price” as the topic; often contrastive (“as for the price, it’s cheap”).
値段も安いです = “the price is cheap, too (in addition to being popular).”

What is the implied subject in それに、値段も安いです?

Context supplies it. From “このレストランは…,” we understand:

“(At this restaurant,) the prices are cheap too.”
If you spell it out, you could say: このレストランの値段も安いです (the restaurant’s prices are also cheap).

Could I just say それに、安いです without 値段?

Yes, in context (restaurants), 安い would be understood as “inexpensive (in price).”
Including 値段 removes any ambiguity and sounds a bit more careful, especially in writing.

Can I say このレストランが人気があります?

Avoid that. 人気がある already contains a が marking the thing that exists (人気). The natural pattern is:

このレストランは人気があります.
If you want が on the restaurant, use a different structure:
このレストランが人気だ／人気です (OK)
人気があるレストラン (attributive) is also fine.

What nuance does 人気はあります have?

人気はあります uses contrastive は on 人気: “It does have popularity (at least).”
It implies a contrast or limitation, often followed by a but-clause:

人気はありますが、席が少ないです。 = It is popular, but there aren’t many seats.

How do I modify nouns: 人気のある vs 人気の vs 人気な?

Most natural/safe: 人気のある + noun (a noun that has popularity)
Very common and natural: 人気の + noun (popular X)
人気な + noun exists but is much less common; many speakers avoid it. Prefer 人気の or 人気のある.

What are the readings (pronunciations) of the key words?

この kono
レストラン resutoran
は wa (spelled は but pronounced “wa” as a particle)
人気にんき (careful: same kanji can also be ひとけ meaning “signs of life,” but not here)
あります arimasu
それに soreni
値段ねだん
も mo
安いやすい
です desu

Why is the topic particle は pronounced “wa,” not “ha”?

It’s an historical spelling convention. The particle is written は but pronounced wa. The same happens with the topic particle へ (pronounced “e”) and the object particle を (pronounced “o”).

What’s the difference between 値段, 価格, and 料金?

値段: everyday “price” of goods/food; perfect for restaurants.
価格: “price” in a formal/market sense (official pricing, economics, catalogs).
料金: “fee/charge” for services, admission, plans (e.g., 送料, 通話料金).
In your sentence, 値段 is the most natural.

Does 安い ever sound negative?

It usually means “inexpensive,” which can be positive. But it can imply “cheap” in the sense of low quality depending on tone/context. To be clearly positive, you can say:

お手頃（な） = affordable
リーズナブル = reasonable (loanword)

To criticize quality, Japanese often say:

安っぽい = cheap-looking/cheesy

Could I combine the two ideas in one sentence without それに?

Yes. Natural alternatives:

このレストランは人気があって、値段も安いです。
このレストランは人気があるし、値段も安いです。 (the し implies “and what’s more/among other reasons”)
このレストランは人気があります。しかも、値段も安いです。 (stronger emphasis than それに)

Is it okay to drop です/ます in casual speech?

Yes.

Polite: このレストランは人気があります。値段も安いです。
Casual: このレストラン、人気あるよ。値段も安い。
Note: i-adjectives like 安い don’t take だ in casual speech; just say 安い (not 安いだ).

Can I use は/が instead of も in 値段も安いです?

Yes, but the nuance changes:

値段が安いです: neutral statement “the price is cheap.”
値段は安いです: topic/contrast (“as for the price, it’s cheap”).
値段も安いです: additive (“the price is cheap too”).
In your two-sentence structure with それに, も is the most natural because you’re adding another positive point.

What does the それ in それに refer to?

In それに, the それ isn’t pointing to a concrete noun; それに functions as a fixed connective meaning “in addition/besides” to what was just said. You can treat それに as one chunk.