Honestly, basically the cost of all the bits at list price bought for the build, plus the cost of all the bits based on current prices less say 5% per year up to 3 years for those items held in your bits box and used on the build, then, with an estimate on the amount of time spent less the time actually spent enjoying building at a rate you think is reasonable re-evaluated based on what that cost is as either achievable or unachievable, all added together and then re-evaluated as what you may have paid for it yourself if someone else had built it, so its a difficult question and without proof of concept ie floated, its price is markedly reduced.
EG (don't know how much they are), years ago at a ellesmere show the southport crew had US frigates / destroyers on show - arliegh's etc, the quality was equal/better to that of PSSHIPS creations, a couple of thousand - I don't know, if someones pride and joy came up for sale and they wanted x amount and the quality and craftsmanship was worth it - then you have to pay the going amount, if it's a billings etc kit, its not going to be in the same league as a GRP hull with a scratch built superstructure, if it is an airfix kit rc'd you would be very lucky to get very much - like an RTR.
So its a matter of what you want for it and what people want to pay, eg the skipjack on offer recently, ideal for me as i'v always wanted to do a sub - but - currently finances are not where they need to be and I've plenty of other projects to complete first - it is clearly worth more than what the seller is offering it for.