I'm sorry, but this is horse manure.
First of all, if we sign Gareth Bale, we get a top class winger. Actually, we get one of the very best wingers in football. There will be no need to sign another top class winger anytime soon if we sign him. Then we have Sanchez (who I think will come good) on the left and Bale on the right. If that's not class, I don't know what is. Ageing class, but class nevertheless.
Second, how is he on the wane? He has a goal or assist every 110 minutes or so this season. A season where he's been less injured than in a long time, but out of favor in Real Madrid. 16 goals and 4 assists in 26 games in La Liga (and he only averaged 70 minutes per game in those 26 games). You'll struggle to find many wingers with better stats than that. He might not be as explosive as he used to be, but that's actually a good thing, considering his injury record. And he's not a one trick pony, only relying on his explosiveness. He can shoot, he can dribble, he can cross, and he's a big, muscular fella who can bully any defence in the Premier League.
Also, money wise, he'll cost a lot. But he'll also generate a lot. You can't only look at the expenses with mega signings. Of course we want recoup every penny, but he'll surely make our economy grow even further.