Katılım ücreti on liraydı.

Because katılım ücreti is a very common Turkish noun compound meaning participation fee.

In this kind of compound:

• the first noun stays plain: katılım
• the second noun takes a 3rd-person possessive ending: ücret + i

So:

• katılım ücreti = participation fee
• öğrenci kartı = student card
• otobüs bileti = bus ticket

This -i is not the accusative here. It is part of the compound structure.

Katılım means participation or attendance in the sense of joining in.

It comes from the verb katılmak, which means to join, to participate, or to take part.

So:

• katılmak = to participate
• katılım = participation

That makes katılım ücreti literally something like participation fee.

-ydı is the past form of the Turkish copula, so here it means was.

So:

• on lira = ten lira
• on liraydı = it was ten lira

More literally, this comes from:

• on lira idi = it was ten lira

In normal Turkish, idi often gets attached to the previous word, and when the word ends in a vowel, a buffer y appears:

• lira + idi → liraydı

So Katılım ücreti on liraydı = The participation fee was ten lira.

The y is a buffer consonant.

Turkish often inserts y when one vowel-ending element is followed by another vowel-starting ending. Since lira ends in a vowel and -idi / -dı begins with a vowel historically, Turkish uses y to make pronunciation smoother:

• lira idi → liraydı

This is very common in Turkish.

Because in Turkish, nouns usually stay singular after numbers.

So:

• bir lira = one lira
• on lira = ten lira
• yüz lira = one hundred lira

You generally do not make the noun plural after a numeral.

This is different from English, where we say ten dollars.

Turkish does not have a direct equivalent of English the.

Whether something is the, a, or just a general noun is usually understood from context.

So katılım ücreti can mean:

• the participation fee
• a participation fee
• sometimes just participation fee

In this sentence, English naturally uses the participation fee because it sounds like a specific fee already known in context.

Yes. This is a very normal Turkish sentence order.

• Katılım ücreti = subject/topic
• on liraydı = predicate

Turkish often puts the main predicate at the end, so this sentence has a very natural, neutral order.

A different order is possible for emphasis, for example:

• On liraydı katılım ücreti.

That would emphasize the price more strongly, but Katılım ücreti on liraydı is the standard neutral version.

Yes. That is also grammatical.

Both of these are possible:

• on liraydı
• on lira idi

The attached form -ydı is very common and natural in everyday Turkish. The separate form idi can sound a little more careful, literary, or emphatic depending on context.

So in ordinary speech and writing, liraydı is very normal.

Because -dır is optional and adds a different tone.

• on liraydı = it was ten lira
• on liradır = it is ten lira / it is presumably ten lira / it is indeed ten lira, depending on context

In your sentence, the speaker is simply stating a past fact, so -ydı is the natural choice.

Adding -dır would change the meaning and style, so it would not match the same sentence.

Yes, it is a very common set expression for participation fee, especially in formal or event-related contexts.

You might see it in places like:

• event registrations
• competitions
• seminars
• workshops
• conferences

So it is a useful phrase to learn as a whole expression, not just as separate words.