Get M520s from Ribble for $25 or less, as long as you get enough other stuff to get over the total order price where the big discount kicks in - usually around $50 or $60.

And if you do go SPD-SL, get the 105 pedals from Ribble, too. They can usually be had for under $50. The 105 is pretty much the same pedal as the Ultegra - it's a slightly lighter silver and IIRC maybe a gram or two heavier. But the bearings and contact surfaces are exactly the same.