As I understood the article's implication here, the objective is to help the magazine accomplish it's impending much larger print run and cover expenses related to that, so perhaps the sum was negotiated along practical rather than symbolic concerns. Which would make sense since it's the easiest to explain and justify.

It's €250,000 ($296,000).

You can ask that question about any sum, making the answer "why not 300K?"

Charlie Hebdo was going under. They'd already made a call for donation in November.

It's not Google directly. Google finance a fund for press innovation (IIRC, that was set up to settle some trial). This found decide to match the 250,000 euros donate by some french press association to arrive to a total of 500,000 euros. (Can't find the article I read this afternoon right now. I'll edit when I found the source.)

The money is needed to be able to print 1 million issues of Charlie Hebdo for next week instead of the usual 60,000. To publish, whatever happened/happens.

Possibly some combination of taxes and internal company policy? ie, they have $x they donate per year, and can get a $y tax write off for donating to a company/non-profit(?) in another country, so they donate $300,000.

Probably:

"We were given a budget of $2m in this department ("innovative press" etc). We want to fund this other N projects in 2015, what can we give to Charlie?"

I think it's to cover the costs of Charlie printing a million copies of the next edition rather than the usual 60,000.