What percent of China's economy benefits from prison labor?

I’m wondering how much they are using and how much they’re profiting from it.

Hard to say. The US’s hands aren’t clean on this count either, though. Lots of telemarketers and newspaper inserters are inmates.

Well China haa not only a lower incarceraionrate than the US, they also have less prisoners:

It is difficult to say anything about the Chinese prison system with total certainty, but probably not much.

First, because not all Chinese prisons are laogai, with a forced labor system. Second, because the manufacturing capacity of China is so enormous, in terms of labor inputs, that the prisoners are but a very small part of it. And third because no matter how forced the labor might be, I would be willing to bet the low productivity makes up for any labor cost savings.

I dunno, but around here I can get a beach mat for under $1.

Are those US inmates forced to participate, or do they opt into those jobs?

I’m sure they opt into them. Still, they enable the “employers” to compete in the marketplace with negligible labor costs, which was kinda the point of the OP. Employers who pay a living wage hurt when going head to head with employers who don’t.