Really depends on the player, their club, their potential, age, squad status and if they're unhappy or something else. Sometimes starting on their current value, getting a counter offer and trying to find good price in between is usually the way to go. If I know it's a player the club is unlikely to sell at first I might start at 1.5x their value and work from there - or times, if they're interested in joining my club and ain't too happy staying where they are, I might start at something below their current value.
But a good rule of thumb is simply asking yourself what you think the player is worth and what he is worth for his current club - and put that up against how important you think he would be for your club. Especially when buying young players with good potential you might as well throw their current value out the window as it doesn't mean anything as sometimes the club and the player himself hasn't accomplished anything so far in their career.