Primarily Yahoo wants to buy small, failed startups with excellent teams for very little money. This is what's known as the "acqui-hire."
These days, it's a common way for big tech companies to hire talented, entrepreneurial people.
Otherwise, Yahoo has a lot of cash available and it's willing to spend big, but only if the target company meets two criteria. It must:
1. Be very strong in mobile.
2. Enhance one of Yahoo's current brands, in particular Yahoo Sports and Yahoo Finance. As an example, this source said that if Turner had not just acquired Bleacher Report for $200 million, it would be the kind of company that Yahoo, under Mayer, would have taken a long look at acquiring.