Song has been a massive disappointment at Barcelona, but that's not much of a surprise. The Catalans play with a single holding midfielder, which Song can't play as. Asking Song to play there, or in central defense, is a recipe for disaster.
Where Song can shine is in a two-man pivot, which is where Song would likely be asked to play at Spurs. Whether it is with Sandro in a more defensive setup, with Paulinho or with Moussa Dembele, Song could regain his form and play a vital role for the club.
Barcelona bought Song for £15 million, but considering his struggles at the Camp Nou, it's tough to imagine him going for near that price now. More likely, he would be roughly a £10 million transfer, which is a good price for a player of his quality. Plus, it would really piss off Arsenal.