Not sure I understand why you are instructed to not include units as this goes against good science teaching practice.
But anyway
if $3.30 /ream
then £3.30/500 sheets
which is $0.0066 sheet^-1 tp four dp.
1.000 oz is 0.02835 kg
0.500 oz is 0.01413 kg or 14.13 g
thus 1 kg of paper is
1 kg/0.01413 kg sheet^-1
=70 sheets to the nearest whole sheet
so 1 kg of paper costs
70 sheets x $0.0066 sheet^-1
=$0.46
Please check the maths and remove the units if you want to.