I tend to agree with Sterling no one can value an aircraft on the present market as it will fetch what someone is prepared to pay and what the seller is prepared to loose to get a sale.
More important to know what it will cost you once the purchase is complete.
Go from the point that you are buying someone elses problem. What appears cheap may not be cheap so do your groundwork and dont end up with nasty surprises and costs.
Ok there will be plenty who are only too keen to take over your purchase and give you fancy valuatons which dont mean much at a cost.
I am not saying dont use a professional to handle your puchase as a good one can save a fortune but to be aware that there are many who are good at talking and only too happy to promise the earth while removing money from you at a fast rate of kts.
Pace