Top pitching prospect Noah Syndergaard would be the centerpiece of the deal, which would also likely include a few other high-level Mets prospects. The talks are thought to be in the early stages, as the two teams have been exchanging names in a possible deal for weeks now. While there has been progress throughout the winter, the financial part of the transaction and Tulowitzki's health are factoring heavily into the negotiations.
The deal is probably still a longshot to get done, as Ken Rosenthal points out:
Tulowitzki has $118 million owed to him over the next six years and the Mets would want the Rockies to absorb some of that money in return for all the top young players they might receive in such a deal. Also, Tulowitzki is coming off hip surgery that ended his 2014 season after just 91 games. Rehab is said to be going very well and he is running now, but he has yet to resume baseball activities.
Although Tulowitzki does not have a no-trade clause in his contract, it is believed that Rockies owners, the Monfort brothers, would not deal him anywhere without his approval. While his stance on moving to the Mets isn't clear, he is known to love New York and has had great success at Citi Field over the years, posting a .438/.534/.833 slash line, and the fences have been moved in again next season.