When it comes to doing rally, you have two different methods.
If the Formation is Broken, remove it from the table, and remove all blast markers from it. Formation is now part of the reserves. It may NOT use the Necron ability to restore lost units as it is not on the table.
If the Formation is NOT Broken, you will make a rally roll as normal. If you succeed, you may remove blast markers OR in place of those blast markers, you may restor Necron Units.
As to the Destroyers, you can beef up the formation by purchasing Wraiths. With their invulnerable save, they make a great screen to absorb shots, and provide the formation with an ablity to do some serious damage in an assault.
I'm a bit iffy about allowing additions, simply because they have a goodly amount of firepower as it is. But feel free to try it out and let me know how it works.
The cost of the tomb complex was based off of a Webway portal actually. The cost increase was because of the expanded power of the Necron portal over the Webway. It's value is that because it's a non-destroyable portal, you can shift virtually any formation still on the table to contest the objective, or counter assault a claiming enemy formation and drive him back.
Also, it allows you to have reserves ON the board, and still use them to strike forward through portals.
So, I think it should cost more than 50 points. But the final cost is still flexible.
