As to the cost, yes Tiagra can be less expensive and are the Ultegras you mention the current 11-speed versions or left over 10-speed? That said, the Gevenalles are only slightly more expensive initially and upgradable at much less cost since all you need is a pair of downtube levers.
The Tiagra and Ultegra levers I mentioned are both 10 speed. As to whether the Ultegra are left over, not sure. I do not know how Shimano runs its factories. Once Shimano changed to 11 speed on Ultegra, did they immediately stop making all 10 speed levers forever? Or did they make 10 and 11 speed Ultegra levers for years side by side? I'd guess the last. Shimano continued to make the old style for years after shifting to the new style to keep stores and shops supplied with parts to fix older bikes. Its probably not a black and white changeover going from 10 speed to 11 speed levers.
As for upgradability, does anyone make indexed downtube shift levers for 9, 10, 11 speed gear systems? I looked on the Shimano website and saw zero downtube levers of any kind. I see MicroShift does make downtube levers for 9 and 10. http://www.microshift.com.tw/road_Shifters.html