$100M is just the amount promised by the billionaire philanthropist. The mission itself would cost several orders of magnitude more.

The article states

"At an estimated present-day price of approximately $10 per watt of laser power, building and operating Breakthrough Starshot’s 100-gigawatt array today could cost as much as $1 trillion."

but goes on to say that the price is plummeting quickly, and so may be affordable in relatively short order.

Okay, but people are gigantic and heavy, and you have to feed them. Also, they usually have to breathe and go poop. They're like hundreds of thousands of grams, and then some, so 70M is less surprising.

Sending a working cell phone into low earth orbit probably wouldn't approach those numbers, I don't think.