I believe it costs 500 Zen (which you can buy with dilithium if you don't want to spend real life money) and is the single most valuable thing you can buy from the C-Store (in my opinion).
The cap increase takes your limit from 10m to 1b ECs, at which point, playing around with items worth 100s of millions of ECs becomes possible... really, it's near-impossible to interact with the Exchange in any meaningful way without getting that cap increase.
Mind you, before you buy the increase, any ECs you earn over 10m will disappear... gone... forever... lost into the ether. So don't sell big-ticket stuff til you have that cap increase.
So do it do it and good luck! (and definitely don't under-sell those ships, unless you sell them to me, in which case, cool, lol).