teisyoku wo tabeta ato de kaikei wo sitara, omotta yori yasukatta desu.

Because あとで is attached to the plain past form of a verb when it means after doing X.

So:

• 食べたあとで = after eating
• not 食べてあとで

This is a fixed pattern:

• Vた + あとで = after doing V

Examples:

• 宿題をしたあとで寝る = I’ll sleep after doing my homework.
• 映画を見たあとでご飯を食べた = I ate after watching the movie.

In your sentence, 定食を食べたあとで means after eating the set meal.

In this sentence, あとで means after.

• 食べたあと = after eating / the time after eating
• 食べたあとで = after eating, then...

In many cases, あと and あとで are very similar, and both can often be translated as after. But あとで often feels a little more clearly adverbial, linking to the next action.

So here:

• 定食を食べたあとで会計をした = After eating the set meal, I paid.

Because 定食 is the direct object of 食べた.

• 定食 = set meal
• を = marks the thing being eaten
• 食べた = ate

So:

• 定食を食べた = ate a set meal

This is standard verb-object marking in Japanese.

In everyday Japanese, 会計をする often means to pay the bill or to check out, especially at a restaurant or store.

So here it does not mean something like formal accounting work.

In this sentence:

• 会計をしたら = when I paid / when I went to pay

A more direct verb like 払う could also be used for paying, but 会計をする is very natural in restaurant situations.

Here, たら means something like:

• when
• after
• upon doing

It often connects one action to a result or discovery.

So:

• 会計をしたら、思ったより安かったです。 = When I paid, it was cheaper than I thought. = After I paid, I found that it was cheaper than I thought.

This use of たら often suggests that the speaker learned something as a result of the first action.

Common pattern:

• Vたら、〜た = When I did V, then ~ happened / I found ~

Example:

• 店に行ったら、休みだった。 = When I went to the store, it was closed.

Because the context is clearly about something that actually happened in the past.

• The speaker did eat
• The speaker did pay
• The speaker did discover the price was low

So 会計をしたら is not a hypothetical if I pay. It is a real past sequence: when I paid or after I paid.

Japanese たら can mean either if or when, and context tells you which one is intended.

思ったより means than I thought or than expected.

Breakdown:

• 思った = thought
• より = than / compared with

So the pattern is:

• 思ったより + adjective = more/less adjective than I thought

In this sentence:

• 思ったより安かった = it was cheaper than I thought

Very common examples:

• 思ったより難しかった = It was harder than I thought.
• 思ったより大きい = It’s bigger than I thought.

The subject is omitted, which is very normal in Japanese.

From context, what was cheap is most naturally:

• the bill
• the meal
• the total price

So 思ったより安かったです means:

• it was cheaper than I thought

Japanese often leaves out things that are obvious from context. Instead of explicitly saying 値段は or 会計は, the sentence simply says 安かったです.

If you made it more explicit, you could say:

• 会計は思ったより安かったです。
• 値段は思ったより安かったです。

But in natural speech, leaving it out is very common.

Because the sentence is talking about a completed past situation.

The speaker already ate, paid, and then realized the price was low. So the adjective needs past tense:

• 安いです = it is cheap
• 安かったです = it was cheap

Since the realization happened after paying, the past form is the natural choice.

This is a standard polite style in Japanese.

• 安かった = plain casual
• 安かったです = polite

Japanese often combines:

• plain verb forms inside a sentence
• with a polite ending at the end

That is exactly what happens here:

• 食べた
• したら
• 思った
• 安かったです

This is completely natural. Only the final part needs to carry the politeness level for the sentence as a whole.

Because in Japanese, subordinate clauses usually use plain forms, even in polite speech.

So before the final predicate, it is normal to use forms like:

• 食べた
• したら
• 思った

Then the sentence ends politely with:

• 安かったです

This is one of the most important things for learners to get used to: plain form inside, polite form at the end is very common and natural.

Yes. That is a very close translation.

A more natural English version might be:

• After eating the set meal, when I paid, it turned out to be cheaper than I thought.
• After I finished the set meal and paid, it was cheaper than I expected.

The key nuance from 会計をしたら is that the speaker discovered the low price at the moment of paying.

Yes.

定食 refers specifically to a set meal, usually a fixed combination like rice, soup, and a main dish. It gives a more specific image than:

• ご飯 = meal / rice / food, depending on context
• 料理 = dish / cuisine / cooking

So 定食を食べた tells you the speaker ate a particular kind of restaurant meal, not just food in general.

Yes, very often.

The たら pattern is commonly used when one action leads to a discovery, especially one that feels noticeable or a little surprising.

So in this sentence, there is a mild sense of:

• I went to pay, and then I found out it was cheaper than I expected.

That matches well with 思ったより安かった. The whole sentence feels like the speaker is reporting a small pleasant surprise.