Everyday there is a bus departing from the Main Bus Station in Xian (besides the Train Station and in front of the City wall). The bus number is 306 and the trip will cost you the bargain of 7RMB (~1USD). The last bus from the Terracota Museum returns to Xian at 6:30pm. If you're at some hotel in the inner city wall a taxi can ride you to the 306 bus station by no more than 7RMB from anywhere.