Since your posting the question seems to me you feel guilty.. So just tell him the cost and tell him why you felt the need to lie and tell him that that lying doesnt solve anything thats why you are telling him the truth now..
He may or may not pitch a fit.. But on all actuality the fit may be more about the lying than the actual cost..
BUT...
you got your Altos so smile...
I too dont volunteer any info but when asked I will tell.. He teases me on my purchases but anything under $300 with cash or debit card I dont need to confer.. its the purchases on the AmEx that i need to let him know BEFORE the bill comes in..