When it comes to selling something, it's worth what people will pay for it. Unfortunately, people don't make purchasing decisions based solely on the cold hard facts. There isn't a formula that you can use.
So, how can you get a ballpark figure? Unfortunately, I think your best bet is comparison. Remember that your buyer probably doesn't have any sentimental attachment, so it may be worth less to them than you. Ideally, you'd compare it to an identical item, but that will be very difficult to find. So, you need to be looking at other kits. Why would someone buy yours instead of the other ones?
If I were trying to sell the thing, I'd make a spreadsheet of similar instruments, and the prices they sold for. I'd then pick a ballpark price, and if I felt I was happy with getting that, I'd put it on the market. If it doesn't sell, I might reduce the price after a couple of weeks. If no one bites, it's obviously not worth that much. If it goes very quickly, it was probably underpriced, but I'd still be happy with the amount I got. Don't waste time worrying about lost money; it's not going to change anything.
This isn't a precise approach , but it should get you somewhere. I would not use a professional appraiser unless I was pretty sure it was worth a couple of grand at least. Otherwise you'll probably spend more selling the thing than it's worth. And, at the end of the day, you can say it's worth $x, but if no one will actually pay you, is it really worth that much?
