And the one rule of thumb that I've seen used is to give the new character 50% of the exp the most experienced character has. (Maybe more if you're replacing the squad leader.)
I've used this a lot. Not in OW, but in a variety of other games, including RT.
The idea is that XPs are a reward for succes - you're not punishing the unlucky guy, you're rewarding the ones who managed to keept their characters alive.
My current OW campaign (in which I am a player, not the GM) builds new characters on the same XP total as your dead character. And when we take in a new player (recently necessary, due to players leaving, stupid RL), they simply get the XPs that everyone else has. Which currently means our new medic has 100 XPs more than my sharpshooter, because I missed an early session and didn't get full XPs for it. Do I begrudge him? Not at all, he (the player) seemed more bothered about it than I was.
In a game as focused on physical combat as OW (atleast our campaign), I can see the point of having characters fairly close in ability. If I was the GM, I'd probably simply hand out the same XP as everyone else had, rounded down to the nearest multiple of 1000 XP. Still an incentive to not just suicide run as mentioned above, without being crippling.
This all being said, I should perhaps mention that I'm a bit bothered by all this insistance on "party balance" and the like. Many of my most enjoyable gaming experiences have involved wildly unbalanced situation, but where the GM (and players) managed to overcome and even use their differences. When we play Ars Magica, Master and Apprentice seems to be a common theme, and those certainly have a difference in power levels!