Microsoft said Monday that it is entering a “third phase” of its relationship with ChatGPT developer OpenAI through a multibillion-dollar investment. But the deal may come with a price for users, too: a $42/mo “professional”-tier subscription that sources say OpenAI is testing among early adopters. In 2019, Microsoft said it would invest $1 billion into OpenAI, then a relatively unknown developer of AI technologies. On Monday, Microsoft said it would extend its partnership through a “multiyear, multibillion dollar investment to accelerate AI breakthroughs.” “Microsoft will deploy OpenAI’s mode…