My guess is WPEngine looked at the amount being asked from them (the lawsuit says "tens of millions of dollars" based on public posts indeed it seems above thirty million a year), divided it by the hourly rate of the best legal representation they thought they can get and decided it's cheaper. They wanted to take no risks. From the other side, after you are sued by Rachel Kassabian you really have no choice but get the best you can find and they thought a former (Acting) Solicitor General of the United States is it. As they say, that escalated quickly.

There's not really a measure to say how much worse an $1000 or $500 per hour lawyer would be but no one is taking chances when you have this much money on the line.

Do you know the phenomenon of "Tournament wages"? Very prevalent in sports, it's why Ronaldo is worth a billion dollars. Because the lawyer's work is only valuable if they actually win, it's a very binary payout, and that drives up competition for "the best".