Actually, the pictured Cobra, is NOT a USFS aircraft. It belongs to the Florida Department of Forestry. And is operated by them. It does not have a 'normal' airworthiness certificate. I believe it is a restricted airworthiness certificate. The reason on that is that this aircraft and others like it, that are operated by government agencies operate under 'Public Use'. Which is a whole different kettle of fish. Legally the aircraft doesn't need a registration, an airworthiness certificate or any inspections. the pilots operating under these regs do not need a medical or pilots certificate. In reality these agencies DO require pilot medical and pilot certificates and do inspections on these aircraft.
Typically these aircraft are what they call bailed aircraft. When they are declared surplus, DOD first offers them to friendly nations, then other Federal agencies, then state governments and finally on surplus sales. The prices state agencies pay range from 1 USD to several thousand.