Because a decent boat mechanic isn't going to just replace the prop seals. He'll put a full seal kit through the leg. New full water pump kit, full lower unit seals (driveshaft, prop shaft, shift rod etc). Everything else gets a check over for things the average Joe wouldn't notice. He's guaranteeing his work so, he's not going to do 1/4 of the job. For my lower unit seal kit, it's $160 plus about $100 for a waterpump kit (that includes a new housing not just an impeller) so that, for me, is $260 in parts. Mechanic labour costs where I live (Australia) are around the $120 per hour. Two hour job makes it a $500 job.