I just wanted to tell everyone how easy it was to get the local bus out to the Terracotta Warriors - and I don't even know how to catch a bus in my own city! It truly is the quickest, cheapest way if you have limited time and just want to go straight to Warriors, and not be on some tour that makes unexpected stops at tea stores, silk factories where you will get "special" price. It takes the expressway, which a taxi might avoid so they can charge you more for taking longer.
Bus 306 does leave from depot just east of the railway station, but it seems to have a stop all to itself, which is actually INSIDE the wall perimeter, from a square which is really pedestrianised road. Just walk back inside the wall when you reach the eastern side of the square in front of train station. You will see a yellow building on a street corner, which has China Post in it (green store). There will be a big busstop sign with a number 5 (I think), and in smaller print is the 306. It leaves very frequently - about every 10min or when it filled, at least in the morning. It will have a 306 on the window next to the door where you get on. Just sit down, eventually someone will come up and sell you a ticket for 5Y one way. It didn't crowd people on - left when all seats were full. It doesn't stop at all in Xian (at least when it was full at 830am), just drives along expressway to an outer town, where it does make a few stops to let locals off - but don't worry, you WILL know when to get off...the big carpark at the end with all the tour buses! The trip takes a while (?40 min to hr) but it was direct and SO cheap, and you will be PROUD of yourself for doing it! I didn't actually take the bus back, arranged a taxi to take me to the museum for 120Y....she took such a long route which I think was to avoid any toll road. Wish I took the bus, and THEN a taxi in town.
I did see a tour for 295Y for a whole day tour, probably not bad money value as entry to the warriors was 90Y without guide, but if time is your most valuable asset, take the Bus! Guides hired on site are very hit and miss at the Warriors - the one I got didn't tell me anything that wasn't in my book or written on a plaque, and she just tried to sell me things.
PS Ride the city wall - it was fantastic! totally alone, but gets a bit warm. A great way to tour the city and be active.