Relevant cost issue

Hello everyone, I am stucking at this question from Mock exam of BPP about relevant cost, the question is:

“Search Co would also need a computerised operating system called ‘Check 2’. The current market price of Check 2 is $12,800, although I Co has an older version of the system, ‘Check 1’, in inventory, which chould be modified at a cost of $5,800. I Co paid $4,500 for Check 1 when it ordered it in error 2 months ago and has no other use for it. The current market price of Check 1 is $5,000, although if I Co tried to sell the one they have, it would be deemed to be ‘used’ and therefore only worth $3,500.

What is the relevant cost of the computerised operating system?”

The answer is $9,300.

Explaination:
If check 1 was not used it could be sold for $3,500 -> $3,500 are relevant
In order to used it must be modified at a cost of $5,800 -> $5,800 are relevant
Therefore, the total of $9,300 (=$3,500+$5,800) are relevant

I think, in this case, ‘Used’ and “Not used” are 2 separately options. So, it should be:
‘Used’ option: Relevant cost is $5,800 (which have to pay for modifying)
‘Not used’ option: Relevant cost is $9,300 (= $12,800 – $3,500)
– Cost for purchasing ‘Check 2’ system: $12,800
– Gain on proceeded of ‘Check 1’ system: $3,500

However, the explaination of BPP above is sum of the figures of 2 options. Why did they do that? Can anyone explain it to me more clearly, please?

Thank you!!