performance issue of query

Hi all, since i m in learning phase of performance tuning i want to know one thing of my query we are having one query whenever we used to check about the cost its showing 18k like that.but after i analyze the table it suddenly decreases into 9 or 8. after some two or three days again it keep on showing the same cost what it shows on the past? can u please anyone kindly suggest me where is the problem is and f.y.i we are analyzing all tables for every week. but dont know where it is getting struked. please