i.materialise is sometimes cheaper, depending on the size of your part, due to the difference in the way they calculate costs from the way shapeways does it. However, I think they only have locations in Belgium and Poland. My recent print with i.materialise was 60% of the cost of shapeways by packing 2 copies on top of each other and connecting them with a 3mm diameter loop. If I only had 1 piece in the model, i.materialise was still 90% of the cost of shapeways. You need to maintain a 0.6mm gap between parts. I think one of the other companies says 0.5mm gap. This is for SLS nylon.