Fact is, even IMO - if Bale has not proven too much, other then showing up the Serie A for being too slow to handle the pace of PL teams (which tbf, has been proven quite a bit over the last decade) - he is going through a hot streak and his star seems to be the flavour of the month - so you're not going to be able to get him for anything less then 35-40M. If he has a quieter "final half" of the season, and Spurs fail to qualify for the UCL next season, you might get him off them for something along the lines of 20-25M - and again you would be very lucky if Levy sold him for that amount, especially to us.
IMO, I'd try and give 1 or 2 of our youngsters (Cleverly, Obertan &/or even Welbeck/Morrison) a chance to prove themselves first, let Bale develop at Spurs a bit (maybe run down his contract a bit more) and if need be, put in a bid for him.
This might backfire of course, he could just get better and better...