I keep a 3 lvl gap on my wizard/cleric character and I seem to keep up with SP pretty well.. then again I'm still lvl 23 lol. So who knows how it will go when i get to 5x and up. Thats when skills get a bit more expensive or at least seemed that way with chinese char's. Anyway ... check this thread it has an sp calculator which tell you how much sp you will get with however many gaps you have for that particular lvl. Also there's a euro char builder which tells u how much total sp you will need at lvl 80 or whatever lvl u chose to calculate..
http://www.silkroadforums.com/viewtopic.php?t=48338