If you properly fair a fixed undercarriage it has a pretty low drag. compared with the additional weight from the structure, machinery and systems for a modern retract it saves significantly in terms of weight and cost.
I don't think so. Referring to a PA28-161 POH I saw a while ago, the Piper cowlings on what were worth about 7kt on top of 100kt - that is 7% which is probably about 15% engine power lost. And this is at the very low speed of ~ 100kt.
I don't think a 10% loss on a much more slippery 150kt plane is an unreasonable estimate. It could be more.
Cost? There I agree.
Weight? There is some extra weight (offset by not having to carry the fairly substantial weight of fibreglass fairings and associated fixing hardware for these) but the fuel penalty in carrying say extra 30kg (actuators, hydraulic pump, etc) is very small. FAR less than losing 10%+ of engine power.