Although collectible, these are usually very common and seldom worth more than a few dollars. Now, nearly every state has beautiful, graphic designs featuring wildlife or nature themes whose proceeds usually go to a charitable cause. The value of these plates has skyrocketed, especially on the internet among new collectors who specialize in these plates. It's safe to say that any new design is worth the most when it first comes out. Plates from certain states are definitely more valuable than those from other states.
Southern states are the most difficult.
Western states tend to fall somewhere in between. Plates from Illinois, Pennsylvania, Ohio, Kansas, New Jersey, Connecticut, New Hampshire etc. Older plates from Mississippi, Louisiana, Oklahoma, Georgia, Texas, etc. This is especially true in California, where relatively common old plates command huge sums of money from car collectors, provided they are a matched pair in excellent condition. I have tried to compile YOM info for each state on my YOM page .