£40m has nothing to do with prices paid in January, it's based on how the market is now. Like you said yourself in the market as it is now 20m only gets you potential. For proven top class talent then you're looking at least 30m, in the situation Spurs are in now though, not having to sell, for the first time in a long while with a real chance of breaking into the top 4 and staying there, then to potentially set them back 3 or 4 years then you're looking at £40m at least because in all probability that's what it would cost them to try and replace him.
People keep saying there's better players out there than Modric, if they mean that in the sense of there's better viable targets than Modric then why aren't the club being linked with them? Why's it only Modric, Sneijder and Nasri? Personally I'm of the opinion that it's because these better players dont exist or aren't viable transfer targets.