As a superstar, Dave will demand a multi-year contract with high wage. Say something like £40,000 a week or about £2 million a year for 4 years. Transfer fee could be anywhere. Swansea paid £6 million to Celtic for Ki Sung-Yueng. The overall investment (transfer fee + wage + others) will easily be in excess of £10 million. To recoup the investment, Spurs will have to play Dave regularly to continue getting that overseas coverage. This would be detrimental to Spurs and its ambition if Dave is not good enough to take Spurs to the next level. We are now better than signing Asian players to just sell shirts. Man United did not buy Park and Kagawa just for commercial reasons. Sure, overseas advertisement, overseas tour, shirt sales and game day benefits are parts of the overall consideration. However, first and foremost is that the player must have the potential to make it at our level.
For David Beckham, the MLS signed him for a lot of money. However, Beckham was also one of the top 2 players for LA Galaxy (the other one is Landon Donovan).