it's worth around 5K-ish according to lassy's guide(but can be worth more depending on how much you want it and since lassy's prices aren't 100% correct all the time)but it can still go for more.
Yeah, if you're looking for that letter, good luck. You really need it. When they go up for sale, they easily go for 15k+. It wouldn't surprise me if it goes over 20k.
You're going by the Lassy guide right for the prices? I was considering putting in some items, but wasn't sure of they'd go over the current HB.