I will do Mew and Machamp for the deck box only then.
I see an auction on eBay, $9.80 plus $10 shipping, shipped from Japan and a top-rated seller. So I can only value the deck box at $20.
According to your post, if Mew and Machamp are about $10 each, then that should cover the box.
You state Flygon ex is $4. The place I see (and trust) this price is Troll and Toad, where it is at $3.50. However, there Mew and Machamp Prime is $18 and $14, respectively. There's even a thread on Mew Prime's value
here. Machamp seems about the same price then. Taking the lowest prices (I've found) into account, 4 + 14 + 14 = 32.
I also see that you want Rescue Energy, Twins, and Seekers. I can trade you 2 each of these, which would be $3.60, $2, and $4 in value based on TnT.
The total would come to just over $40, which based on the Leafeon/Glaceon deck box I have found and the supposed price of the Walking Starter sleeves of yours (I found one at $18, but I'll take your $20 value) should be equal, even slightly in your favor.
Yes, I researched thoroughly. Though if you say no to the trade then alright.