...Why are they so cheap?
And, I have a cruise planned in June, so I can't go then, when all cruise lines have them for $299, why do they increase to $400-$500 in July and August?
Okay, pretty much all I wanted to know, is why were they so cheap in the first place...

But thanks to everyone who has answered!

"because july and august are summer vacations for schools, so more people will be willing to pay little more for it. Its simple supply and demand economics stuff."
(chosen answer)