Transfer price

Hello SIR
Hope you are fine (as always)
i have lil confusion in one question
Q details;;Division A sells (special ingredient Z) to Division B which division B work further and produce product MADISON sells @ S/P $115

Cost card of product Madison
Total Demand=2000
Raw material 8.5kg @ $2/kg
Special ingredient Z: 2kg/U (transfered from division A)
Other variable cost are 60% of selling price(s/p=$115/U)

Cost card of Special ingredient Z
Transfer price=$15(same price charge externally)
Variable cost=9
Fixed cost=3
Saving on internal transfer=1.5
4000 spare capacity (total requied by division B as 2/U*2000demand=4000)

Q;Calculate the impact that a decision not to purchase ingredient Z will have on the profit of the group??

well i think the loss on company profit is just fixed cost because its spare capaciity
So the company profit is reduced by 4000*3F cost =$12000

but book not show the same ans as
Impact on group profit is
Lost revenue=2000*$115=230000
Saved cost
Division A Vcost=2000*(17+86)=172000
Division B Vcost=4000*7.5=30000
Lost contribution to group=230-(172+30)=(28000)

Sir why they reduce group profit by lost contribution even this is only forecast..actual demad may be over or understate..??
but fixed cost is very obvious coz is dont make we loss 12000
Am i right??