Learn Farsi: این کتاب یازده تومان است.

In Farsi, nouns that come after a number are always kept in their singular form. So instead of adding a plural ending, you literally say "eleven book" or "eleven Toman" (یازده تومان).

I thought the currency of Iran was the Rial. What is a تومان (Toman)?

While the official currency is the Rial, Iranians almost exclusively use the تومان (Toman) in everyday conversation and on price tags. One Toman is equal to ten Rials. It simply drops a zero from the price to make large numbers easier to manage!

Why is the word است (is) at the very end of the sentence?

Farsi sentences usually put the verb at the very end. While English uses a "Subject-Verb-Object" order, Farsi places the verb last. So, this sentence translates word-for-word as "This book eleven Tomans is."

How would a native speaker actually say this sentence in casual conversation?

In spoken Farsi, تومان is usually pronounced as تومن (tomen). Also, the formal "is" (است) often shortens to an "e" sound attached to the previous word. A native speaker would likely say این کتاب یازده تومنه (In ketab yazdah tomane).