How do i travel from xian to Guangzhou by bus because i want to take my travel as cheap as i can?

As I know, no direct long-distance bus runs between these two places. You need to change another transportation.
It's suggested to take high-speed train from North Railway Station, get off at Guangzhou North/South Railway Station. They depart at 08:03, 09:53, 10:54, 11:28, 12:29, 12:35 and 13:05. The trip takes 8 - 9 hours. The ticket fare of second class seat is CNY813.5 and first class seat is CNY1301.5.