I don't know if this is true for all levels, but in the 50's the xp needed for each next level increased by 100, so I was able to figure out when I would get to level 60 to buy something.
So, for example, L49 to 50 was 1,000 points; to 51 was 1,100 more, 52 was 1,200 etc
Or, L49=10,000; 50=11,000; 51=12,100; 52=13,300 (these aren't the actual numbers)
Somebody who's good at math could give you the formula for the progression