So you get 50xp for sticking yourself with a needle after the medic gets how much for giving you the needle and then he gets how much more once you stick yourself with the needle?
Yep. I consider 50 XP a fair reward for not making the Medic waste his time. You probably get more if you revive yourself while in close proximity to an objective, too.
And besides, if everybody gets it, then it's not imbalanced. "You've survived long enough to get revived. Here's 50 points."