Tourism is FREE once you get the tourism tech. Get that ASAP. If you like to warmonger right away, keep an eye on the ever increasing value.
As far as improvements, forget about it. Its not balanced well, the improvements dont even pay for themselves, unless you park the building on a huge tourist terrain buff....or you have a lot of influence already. Or if you have lots of global tourist buffs, it just might break even. But if you have a LOT of influence early-mid game from being a galactic dik, yeah, it may pay off early. So, no matter your situation, if that first tourism building doesnt even pay for itself, ignore the tech tree and dont buy tourism buildings.
For late game, I'm sure its worth it, as you would/should indeed have that influence, but by then, its usually just downhill mopup.
