This could be something that you bought for a higher price than what most people would guess based on the item, or it could be something you bought for a normal price that has gained significant value as time has gone on.
What made me think of this question is a LEGO minifigure I got with my “The Hobbit: An Unexpected Journey” disc. It is Bilbo Baggins in a blue coat that was apparently only sold in that movie box only at Target stores. Even considering the exclusivity, I would have guessed maybe $10-20 for such a tiny piece of plastic, but there are sold listings on eBay from $80 to $225. I could possibly even get towards the higher end of that number since I still have everything in the original box in good condition. It’s not worth a ton compared to some other items people may own, but I think most people would not expect nearly that amount.
I agree something like that shouldn’t be given up. It’s amazing that it’s made its way to you and still works. While they don’t take up nearly as much space as your plane does, I have a deed, some bank statements, and some letters from relatives that are 100-150 years old. They wouldn’t be worth much to anyone but my family, but it’s great having that history to hold onto.
A number 1 plane is actually tiny. That’s part of why they are expensive. They are small enough that demand was low since most people didn’t want/need them. They only really fit in child hands.
Now they are a collectors item since people want to collect them all, but they never made many.