I think it may some times fluctuate +/- ~1m, unless jagex releases something to make it stronger/more useful or weaker/less effective; I would buy it now, get all my use out of it, and then sell it to minimize loss.
It's going to fluctuate. There are like 10+ killing the boss on every world. It dropping in price is inevitable. If you plan on keeping it no matter what the price will be, then sure go for it.