in general the skill up rate is roughly the same between 1 - 250. (note there will be stretches where you hit hell areas with 300 combines between points and in general this point is 170 - 210, everyone hits it at some point, but not everyone hits it at the same point)
note, some skills go up faster than others..
again.. all this info is available at
www.eqtraders.com go check out the forums there for questions..
and yes, in general it seems like its harder to get skill ups as you go up in skill level.. generally because combines are more complex, have more sub combines, require more farmed items/imbued items, etc.. in general the higher you go in a skill, the longer it takes to prep a final combine - either imbue time, sub combine time, or farming time.. (jewelry is one exception as everything is storebought, it just gets more expensive)