As best as I can figure from the published official government taxi rates, the official rate is $8.800 (which is about US$4), i.e. centro to terminal de transportes.
Apparently a board at the entrance to the port shows a price of either pesos $15.00 (which is US$7.50) or pesos $20.00 (which is US$10) depending on whom you believe.
I am not surprised that some people would see a sign with $20 on it and wrongly assume that it is us dollars when in fact it is pesos.
I even read that some taxi "supervisor" at the port entrance is urging people to accept a price of $40 per person!
As far as time and distance are concerned, I've also seen widely different numbers but a simple look at google maps will show that the cruise pier is no more than two miles from Plaza Bolivar.
So would I be correct to assume that a reasonable price to pay for the taxi from the cruise port to Plaza Bolivar is about US$10?
Thanks to all for helping me figure this out.