The Orb and Galor and Jem'Hadar Attack vessel are popular (and expensive) because they have a much, much lower drop rate than any other Lockbox ship. The Galor and JemHadar are even more expensive now because they do not give out Cardassian Lockboxes anymore, meaning that they are getting rarer every time someone sells on the exchange.
Add to that the fact that the Mirror Cruisers are simply an Assault and Star Cruiser in different skins and slightly different layouts, and you have the makings of a cheaper ship. Everyone wants the one that looks different, even if it doesn't always play differently.
Also, a master key may "only" cost 125zen, but you can also make about 1 million EC in less than an hour, either by selling high-demand common items on the exchange (Duty Officers) or moving quickly in the Universe Tour event (you can make 3 million or more in an hour with the right route and speed-boosting gear). EC is cheap, Zen is less cheap.
And yes, you don't need to pay a lot of real money for good items (good ships is another question), but some people feel a few dollars is worth more than the time they would spend grinding dilithium.
I could spend 3 months making the 500,000 dilithium I'd need to buy enough Zen to pick up the Tactical Escort Refit (for a new skin and quad cannons), OR I could decide not to go out for lunch tomorrow and just buy it. Decisions!
*As an aside, please make the effort to spell better, I could barely read your post. It's understandable if English is not your main language, but legibility is key unless you are four years old.*