The travel agency that makes the arrangements for these trips is owned by the JW. They do not do it for cost. They make a profit and it is substantial. Just as in the arrangements for hotel rooms for the conventions held every year, there are freebies for some in the WTS who are part of this. Details on this I don't have.
If you compare what a trip like this would cost from a non-JW vendor, are the prices competitive? I don't know, it has been a long time since I went on one and the rules have changed.
As to attending conventions you are assigned to, why do you think the individual JW has to go to the secretary to get the hotel information for those other conventions? It is a control mechanism. They don't want JWs streaming to Hawaii or the Bahamas for a DC. It interferes with the rooming arrangements and the publications available to hand out (used to screw up the food arrangement too).
*** km 12/04 p. 4 2005 "Godly Obedience" District Convention of Jehovah’s Witnesses ***
Another Convention: To ensure that there is sufficient seating, literature, rooming accommodations, and so forth, publishers are encouraged to attend the convention to which their congregation is assigned. If circumstances require that you attend a different convention, please see your congregation secretary for the mailing address of the convention you will attend, which is printed on the back of the Special Needs Room Request
*** km 1/84 p. 5 1984 "Kingdom Increase" District Convention ***
District conventions are arranged at many locations to reduce travel. In nearly every case, the location assigned for you to attend is the one nearest your congregation. Seating, literature, food orders, and so forth, are planned on the premise that the majority will attend the convention to which they are assigned. However, if a few for important reasons must attend a location other than the one assigned, they may ask the convention coordinator in their congregation for the address of the convention headquarters where they wish to attend