The Magnaflows have been out for awhile. I've seen them for around $650 each side, but that's still way too much money, IMO. However, it does look like it would be a less restrictive system than the stock exhaust manifolds. Doug Thorley makes a nice set of headers, but they only make them in long-tube style, which deletes the front converters. Most that use the Doug Thorley don't run any converters and use CEL eliminators at the rear O2 sensor locations. Those run $473/set, which is a little more manageable.
https://shop.dougthorleyheaders.com/collections/2003-nissan-frontier
https://shop.dougthorleyheaders.com/collections/2003-nissan-frontier
