The recent death of Pernell Roberts, who played Adam, has a lot of people wondering about the value of their Bonanza autographs. The cast actually signed quite a lot of autographs during the original run of the series. Those who held on to those autographs have valuable memorabilia.
Adam was the first of the original cast to leave the series, and the only one to leave of his own volition (Dan Blocker died during the show's run). Roberts considered himself a serious actor and hated signing autographs. So even though he outlived the other Cartwrights, his autograph is going to end up being as scarce as the rest of the cast, simply because he didn't sign much after leaving Bonanza.
Demand is high for signed cast photos of the original four. There are many, many more preprints than original autographs floating around. So be sure you have authentic autographs.
An authentic signed photo of the original cast is worth $1000+ today, and values are rising.