There are Ttwo types of target materials one is pitch based and the other is sulphur based. The old standard is pitch based which is by product of petroleum. As petroleum prices rise so does the cost of pitch which in turn drives target costs higher.
Prices on sulphur based targets have also seen price increases but at a slower and lower rate of increase keeping prices more stable. Sulphur based targets are being marketed as biodegradable as this material breaks down at much higher rate than pitch based targets.
Sulphur based targets known as Bio targets have been around for a long while but have not gained acceptance from the shooting community as they don't smoke ball when hit hard like pitch targets.
Sulphur targets are becoming more common as higher pitch prices and its availability are driving more shooters to sulphur as lower cost alternative.
They've always been cheaper, but that doesn't answer your question. WF has always pretty much set the standard for things in the industry. Rick Lawry has done a good job resolving the pitch issue and he has targets ready to ship at a good price, everything being relevant that is.