Agreed.
Overall, Modric is probably a slightly better player than Moutinho - even though they're not exactly like for like.
But Moutinho has a few qualities that would make him, in many respects, more valuable to Spurs than Modric. First and foremost, he knows (and loves playing for) AVB. He knows how AVB would want him to play. He would love to play for AVB again. He is very keen, therefore, to play for Spurs.
For all these reasons, the signing of Moutinho would massively help AVB to settle in properly and to get a good first season under his belt. Crucial, given what happened to him at Chelsea.
So spending all but £5 million of the Modric fee on Moutinho (who is, let's not forget, a year younger) would be perfectly justifiable, IMO.