You can't really compare the spell books to runics because spell books have a high chance of having no properties, and those that have properties rarely have more than 1 or 2. Runics always have magic properties and always have an expected average number of properties. Another difference is that Spell books only have a few possible properties while weapons have 33 possible possibilities.
All that being said, your odds are about 1 in 1,000 of getting a super slayer property* (undead, reptile, arachnid, demon, elemental, or repond) on a runic-crafted weapon. Since a Bronze Hammer always gives three properties, you should get a Super Slayer every (1,000/3) 334 properties or one every (334/35) 10 bronze hammers. That's about my average. I've gone as many as 30 bronze hammers without a SS, but I've made two with the same hammer before as well.
If you are looking for a specific super slayer, then your odds are 1 in 6,000.
And yes. Runic-Made Super Slayers are rare and valuable; especially since they made the intensity change to them earlier this year (i.e. all runics can hit 100% intensity). Besides the rarity of the property, the fact you can choose the weapon you make (i.e. fast and high dps) also makes them much more valuable than a typical looted weapon.
* 33 possible Properties of which Slayer is 1. About 1 out of 30 slayers rolls as a Super Slayer. 33*30 = 990 (close enough to call it 1,000).