The experience, E, required to achieve level n from level 0 is:
E = 3.5 * n * (n + 1)
That's the formulae I found on the wiki. So, If I replace N with 50 my answer for the amount of experience is 8925.
Well, every hostile mob in the game gives 5 experience upon death (also wiki sourced info). That being said, 8925 divided by 5 is 1785.
Something can't be right, I've reached level 50 many times thus far, and there is no way I've killed that many mobs per level 50 obtained...
Edit: Ok, Now I'm 110% sure something is horribly wrong. When I checked at the time the comments were all made on this post my in game statistics recorded 2,160 Mob kills. I just checked again, out of curiosity and it now states I have 5,317 Mob Kills. Thats over 2,000 kills somehow gained pretty much overnight. Somethings seriously not right, is the statistics page bugged?
Second Edit: Ok apparently the statistics page will never be correct for me, partially because it changes if I play offline (which I do about 70% of the time) and mostly because it doesn't collect its data for each exclusive world individually. So short of manually counting each monster I kill I dont think this question will ever be answerable unless I completely delete all my statistics folders and create a single game for the sole purpose of figuring this out. If someone has any ideas other than that I'd love to hear them.
Third Edit: The bounty description is a lie, I am playing Vanilla SSP not SMP.