10% would be more typical. Perhaps the math is that OpenRouter inside Stripe makes it twice as valuable. You often see acquisitions priced on the value of the company post-acquisition.
The 7 billion figure is not just open router's current value. It's also including part of it's projected value in the future. So that 7 billion includes current value + value say 5-10 years out.
Remember when Google acquired YouTube for $1.6B in 2006? That amount seemed ridiculous at the time. Its now peanuts compared to YouTube's yearly revenue.