Many years ago, I assisted with the creation of a 'rivers trust' to help restore the natural habitat of a counties rivers. At a Rivers Trust convention I attended, there was a fascinating and well researched paper presented that conclusively showed that each salmon caught on Scottish rivers was worth £2,400 to the local economy. The researchers had interviewed several hundred visiting anglers who had completed comprehensive questionnaires, and the results showed that many came with none-fishing family or spouses, all stayed in hotels or lodges, ate in restaurants, purchased gifts, hired cars, brought petrol etc. As a result, the rivers habitat was invested in, fish stocks increased, Scottish salmon fishing was promoted internationally and the tourism expenditure input into the local economies increased exponentially.
As Ian says, precise figures for this sort of benefit are impossible to obtain, but Scottish tourism gained a proven massive increase with investment in the Salmon rivers. Surf Snowdonia, Zip World, Eirias Park and other local attractions all contribute collateral income to the local economy, but precisely how much can only be generalised. Whether the cost benefits of Eirias Park are worth a 'loss-leader', is difficult to establish, but I suspect that attendees visit and return concert venues without putting a significant benefit into the local economy.