FWIW, I read this thread and used the link for cruisecompete. As I wrote in another thread, we are planning our first trip to Greece and our first cruise.
Three TAs have answered so far. The prices ranged from $16,000 to $28,000 per couple! BTW, the fares, directly from Silversea, were just over $7,000 per person or $14,000 per couple. That included a 60% discount and free airfare.
I guess the only conclusion is that if you use a TA, you better also do your homework! I was pretty stunned.