not a teacher
The Windsor Castle are very beautiful. They are worth the cost of entry alone
Your sentences are both incorrect.
Windsor Castle is very beautiful.
They are worth the cost of entry alone.
It's not clear what "they" refers to. However, I Googled and found this phrase in a tourist's comment about visiting Windsor Castle.
"…liked everything, the inside rooms and painting collection are worth the entry price alone."
This means that although there are many marvelous things to see in Windsor Castle, the tourist would have been happy to pay the entrance fee just to see the inside rooms and painting collection. In other words, the rooms and paintings were, by themselves, worth the entrance fee.
It's worded in many different ways but it's a common expression, used in all sorts of circumstances.
"I enjoyed the match. There was some good football in both halves but that clever first goal was worth the price of entry alone", or "…worth the ticket price alone" etc.