I don't think he's going to fetch Spuds a Top 5 all time transfer fee (adjusted for inflation). Most of the players who commanded the highest fees were the best players on the planet at the time of the transfer. While Bale might be on the best form of any non-Messi/Ronaldo player on the planet right now, he's never been a very consistently elite player until this season.
Kaka, Ronaldo, Zidane all were considered the best players on the planet when they were bought by Real.
Bale will probably fetch Tottenham a £50 million fee, about what Chelsea paid Liverpool for Fernando Torres. List of Highest Transfer Fees for science