Countries whose cities host the Summer Olympic Games increase significantly their success during the competition. We study whether such effect is lasting or not. We compute the effect of hosting on the total number of medals in the subsequent games. To confront the issue that the selection of the host city is endogenous, we use a natural counterfactual: countries whose cities also bid for the Olympics but were not selected by the International Olympic Committee. In all cases, we find that Olympic success on medals fades away immediately after hosting.
That is from this paper by Jose L. Contrerasa & Alejandro Corvalanb. (Economic Letters, February 2014).
There is also evidence that ". . . countries hosting mega-events tend to over-estimate the benefits and under-estimate the costs" as indicated by financial returns of companies involved in Olympic Games - and that evidence is from Beijing.