Wow, a colossal waste of money. Space is far, far too vast and the time scale of human civilization far, far too small. To think of what other good uses that time and money could be spent on (that goes for SETI too)
Firstly, it's not a waste of money because it'll be spent on employing people, building things, developing technology, etc. They're not just pouring money down a hole.
Secondly, although it's hugely unlikely they'll find anything, in the event that they do it'll fundamentally change our perspective of who we are and where our place is in the universe. The outcome, while improbable, is so huge it's still worthwhile trying.
This isn't a direct comment on Hawking's proposed search, but there are things that would have real value for space exploration and life on Earth - new energy sources or efficiencies, life extension, AI, production methods, etc. If we one day want to send von Neumann probes out, we'll need to master these fields and none would have uses isolated to space exploration either.
Yes, space is vast but once upon a time the ocean was too.
Secondly, although it's hugely unlikely they'll find anything, in the event that they do it'll fundamentally change our perspective of who we are and where our place is in the universe. The outcome, while improbable, is so huge it's still worthwhile trying.