Akrapovic exhausts are extremely well made and highly regarded exhaust systems. Their performance and sound on the Honda XR650R are both excellent. They are quality made and well supported. The price however is about double any other exhaust system on the market. They are extremely expensive.
There are dozens of exhausts available for the Honda XR650R. The Flow brand (re-branded) exhaust is one of the cheapest around. It gets mixed reviews from many people regarding it’s build quality and performance. The general consensus is that it should be avoided. But if it was dirt cheap I might be tempted to give it a try.