Relevant cost for non-current assets

I posted this question in 2018 and I got explanation from this question which understandable but I will also post two similar questions but those questions confused me

a machine is no longer used by a company.it could be sold now for net proceeds of $300.its only other use is on a short-term contract which is under consideration.the variable running cost of the machine during the period of the contract would be $400.on completion of the contract the machine would have no realisable value and would cost $150 to dismantle and remove

what is the total relevant cost of using the machine on the contract?

loss of proceeds-300
running cost-400
dismantle-150
equal 850
could u explain it why i take 300 as loss of proceeds?

Your explanation which is comprehensive and clear because the asset has no spare capacity and so is not available for other uses for other contracts
I
t is an opportunity cost (i.e. lost income).

If they did not use the machine then they would sell it and receive 300. By using it, they will not receive the 300 and it is therefore effectively a cost of using the machine.

Similar two questions

1) A machine owned by a company has been idle time for some months but could now be used on a one year contract which is under consideration. The net book value of the machine is $10000. If not used on this contract, the machine could be sold now for a net amount $1200. After use on the contract, the machine would have no saleable value and the cost of disposing of it in one year’s time would be $800

Opportunity cost now—-1200
cost of disposal in one year’s time-800
total relevant cost====2000

2) Ace limited is considering a new project that will require the use of currently idle machine. The machine has a current net book value of $12000 and potential disposal value of $10500 (before $200 disposal costs) and hence has been under depreciated by $1500 over its life to date. If the machine is to be fit for purpose on the new project it will have to be relocated at a cost of $500 and refitted at a further cost of $800

Opportunity cost now (10500-200)—10300
Relocation cost—500
Refitting cost—800
Total cost =11600

1)- in this question we can see that its net book value is 10000$ but its realisable value is $12000 which is understandable

2)- in this example the machine’s net book value $12000 but its realisable value is taken (10500-200)=10300 but in fact the company would lose more why we ignore here the net book value instead of 10500?

If we think it from FA and FR point of view, despite its book value is 12000 and it is sold at 10300 it will be loss on diposal 700 but from management accounting point of view is 700$ here not considered as relevant cost?

could you give a reasonable explanation?