I don't think you're in a bad position at all
for APS-C, you have:
* the 18-55, which I'd use as a 18mm f/3.5 prime
* the 24-70, as a general use zoom... that's lacking on the wide end but quite long in exchange
* the 50mm f/1.8 as a low light lens, probably a bit longer than I'd like, but I'd go for the 5D2 for low light anyway
* the 55-250, for even longer reach
for full frame, only:
* 24-70, a great general-use zoom
* 50mm f/1.8, a great low light normal prime
So, for APS-C you don't have anything ultra-wide, but apart from that you're fine. But for full frame, you're lacking on both the ultra-wide end (<24mm) and on the long end (>70mm).
A 70-200 f/4L IS and a Samyang 14mm f/2.8 (caution: only manual focus) would fill both gaps, and will fall just within your budget if you sell the 55-250.
In any case, for me the biggest question is... why do you need the 550D?
I'd sell that one and its two lenses. If you want a smaller camera to bring around, then buy something like a Sony NEX-5N with Sigma 30mm f/2.8 (much better camera, and much smaller too).
(actually, I'm halfway there: I already have the 5N, the sigma is coming in the following weeks)
BTW: the Samyang 14mm is not perfect but it's a very nice budget lens, read my comments about it here:http://www.similaar.com/foto/equipment/us_lensc.html#samyang