The odds of getting 1 no. right is 1/50. The odds of getting a 2nd right is 1/49 ('cos 1 ball's already gone), etc. down to 1/46 for the 5th no. (I don't think there's anything special about the Bonus no. - it still comes from the original 50, doesn't it?)
Multiply all those = 1 in 50x49x48x47x46 = 1 in 254,251,200
The 6th no. may be any of the 45 remaining, so you have 45 ways of winning. Divide by 45 and you have a chance of:
1 in 5,650,027