Repond is a super slayer, they are worth a whole lot more than a regular slayer book. Extra mods can also really affect the value on both regular slayers and especially the super slayers.
I think the super slayers you can get as a property on books are:
Repond
Undead
Elemental
Arachnid
Reptile
On pricing, I have no idea. You can go through many thousands of scrolls and not ever see a super slayer. The only one I have ever gotten was repond slayer with MR1, and I sold it for 1.5M. That was high I think, but the person I sold it to really wanted it, no one else had one at the time, and I really wasn't eager to sell it at all.
I have no idea what a SCRAPPER'S superslayer would be worth...that would be something to see!
-Skylark
P.S. I have given away dozens and dozens of regular slayers of various types with & without mods...seems like the ones people will actually pay for are things such as dragon, blood elemental, poison elemental, and scorpion, but at most they sold for a few thousand.