Creaking floorboards?

the house i want to buy has creaking floorboards. it was newly updated. the owner installed a downstairs wood floor by hand, im told. if that is so and there are so many updates, why didnt he fix the creaking floors upstairs? btw he did put new carpet over them. should i be concerned? the agent said that i could have an inspection to see if it would interfere with the structure of the home? i think she said that by accident. is it possible that something more serious than a nail could be wrong? she said she didnt know. also how much would it cost to have this fixed? thank you. :)