ExpertFlyer.com provides the base fares although one time I was caught out because the AA fares hadn't caught up with a recent increase. Just enter candidate cities you're willing to travel to, remembering to specify the purchasing city to match the starting and ending airports. (Be sure starting and ending airports are the same - that triggers the RTW price lookup.)

Note that 'best starting point' depends on the class of travel you select and usually to a lesser extent the number of continents.