That would be the electric motor usually

When the repair cost starts to get past 50% of the value of a new replacement pump

Motor capacitors, mechanical seals, motor bearings, pump lids, O rings and corrosion - also not forgetting the "Run Dry" scenario and air ingress.

This varies from brand to brand. You should get in excess of 5 years of trouble free operation if you carry out a little preventative maintenance every 6 months or so. Basic checks are required. In the perfect world, every pump should be housed in a dry ventilated weatherproof enclosure. Exposure to the elements and or moisture results in Corrosion ageing your motor/pump external metal components prematurely. Most Wooden boxes located outside that house pool equipment are not totally weatherproof. This is 1 of the biggest problems a new pool pump faces in its life. On occasion it nearly impossible to dismantle a pump due to holding bolts fused and corroded very badly. Remember! ... you buy Cheap, you get Cheap!!

This varies from brand to brand. You should get in excess of 5 years of trouble free operation if you carry out a little preventative maintenance every 6 months or so. Basic checks are required. In the perfect world, every pump should be housed in a dry ventilated weatherproof enclosure. Exposure to the elements and or moisture results in Corrosion ageing your motor/pump external metal components prematurely. Most Wooden boxes located outside that house pool equipment are not totally weatherproof. This is 1 of the biggest problems a new pool pump faces in its life. On occasion it nearly impossible to dismantle a pump due to holding bolts fused and corroded very badly. Remember! ... you buy Cheap, you get Cheap!!

Very difficult to estimate accurately, as there are to many unknowns that we face. We can give you an idea. Pump spares vary significantly in cost