That does suck , and Wrex probably is dead if you don't import him since a new Krogan is introduced in the game.
That's one reason i wait on price drops. I have a harddrive w/ my 360 but very little storage space on it. It will only get worse though as the next generation of consoles could be download only systems.
You may be able to buy used 360 harddrives at local game stores since alot of people bought larger ones and transferred their stuff onto the bigger drive.