Wiz,
it is a difficult question to answer because there are a lot of variables.
The winglets provide a 3-5% improvement in burn, increase optimum altitude by approximately 2000' and increase second segment climb weights by about 2 tonne.
On the shorter sectors the better fuel burn doesn't make a huge difference, but on a MEL-PER it makes about 500kg difference.
Again on the longer runways it makes little practical difference, but on shorter runways it allows you to use Flap 25 which normally shortens up the field length, but kills the climb limit weights. The winglets recover some of that loss.
Finally it depends on where you bought your winglets from. Aviation Partners Boeing will (or used to) sell you winglets cheaper than Boeing will.
The cost of the winglets (+ installation) will alter the breakeven period over which you need amortise the costs.
I have not done the sums recently to work out the stagelength where the winglets breakeven - but I hope the above helps.