Is XIAN expensive to visit and how about hotel/guesthouse?

I have been there once. I do not think visiting there is expensive.
As for the tickets price for most of the attractive spots, the most expensive is about 150 RMB and the others are about 50-100 RMB.
For the hotel rate, it is hard to say. There are hotel with about 150 US$ per night, while some hotels have the price at about 50 US$ per day.
I advise you to find a tour guide. Although it is cheap without a tour gudie. But there are many attractive spots with long history.

Hi. Did you buy the air Asia ticket to xian? I am also going in may on my own. This is my first trip to xian. Do not know what to expect.

Do you plan to go to the city from Malaysia?
Well, if so, you may need transfer in Guangzhou and then catch an air to that city.

You are right. Air to Guangzhou is operated by Air Asia .