In general, since the silver ones are unreleased prototypes, and much fewer in number, they are also more expensive. I had sold a black one with good screen for the same price, and another black one in almost perfect condition with box, manuals etc for about 700Euro. Now, I have 2 silver ones only.

For comparison, a silver one with dot on the screen (not mine) was sold about 2 months ago for about 500USD, while a guy with black ones with dots, he has them for sale at 150USD for a few years and nobody buys them... Black ones are worthy only as complete and in good condition and with clean screen, while the silver ones are much rarer so even with a dot, they still have some value.