your plate is worth whatever someone is prepared to pay for it.
certain types of number get higher values, like dateless plates, ie ones with no leading or trailing age identifier, like abc 123
a non dateless plate would be like abc 123W or A123 abc
if your plate spells something or has well known initials in it its worth more.
I wouldnt trust any company that values it as they are wanting to sell if for profit so will under value
do your research for similar plates and sell at what you are happy with