This is a business law question

Sixteen-year-old Brent's parents threw him out of the house and told him he couldn't live there any more. He has lived on his own for two years an d his parents do not contribute to his support. Brent contracts with Dave for the purchase of a new mattress for $150. Dave delivers the mattress and sends Brent a bill in the mail. In the meantime, Brent finds out the reasonable value of the mattress is only $100. Brent feels that Dave took advantage of him and refuses to pay, Dave sues him for $150. Assuming the bed is considered as necessary, what amount. If any is Brent legally oblig ated to pay to Dave