TransferMarkt.uk currently lists his value at 3.5 million Pounds or about $5.4 million US. $8 million would be a significant overpay for the Mexican national team player if that's the transfer fee to Mallorca. His current deal with them has three years left on it.
Last year dos Santos' agent said the MLS is no place for a twenty two year old player, in an interview where he also said dos Santos was likely to stay at Tottenham.
With all MLS silly season rumors, it's important to remember MLS (and China) for soccer football agents is like Los Angeles to NFL owners. It's a magical mythical place they can threaten to move to when trying to find an offer for their client.
The thing that makes this deal stand out is the relative youth of dos Santos, as well as the fact that he's still got plenty of time on his contract.
The Galaxy of course do have a designated player spot open, and if they were to land dos Santos it could be a huge splash both on the pitch and in the stands.