If they view it as a business, then viewing the st overruns as a sunk cost makes sense, if they view it as a hobby then hobbies have budgets
The author doesn't understand what a sunk cost is, it means already incurred.
In this case, they may be committed or feel they have to spend the money but haven't yet - thus it may in fact be relevant to other costs if they want to shift funds from other spending to mitigate the overrun and remain neutral in terms of cash outflows.
