Slayer heroes are only there to deal with big, tough Behemoths in combat, or killing that bad-ass Lord. Ignoring how he gets there of course. So I wouldn't compare cost to warriors or anything else. Not saying this is a good deal...but focusing on the role of "killing that monster that will otherwise eat my entire unit of infantry". The idea in theory would be hold that monster in place with an infantry block while the slayer character sprints desperately across the map over three turns.
Smithing is dumb actually since he's already got D3 against Behemoths. So some 50 point combos: Really wanted to see if MR Flight would work for casual games, but I don't think you'll keep the D3 wound rule, or the Slayer rule on that attack. So forget it.
MR Alaric the Mad, Rune of Speed: Ignore all armor saves, put it on a handweapon, Init 5-6 (or 7 for 55 points on a Deamon Slayer) isn't too bad.
MR Swiftness, Rune of Fury: Ulrock's build above. Maybe on a hand weapon with Cleaving, but I'm thinking like Ulrock and using a GW for some built-in AP and attacking at base I isn't awful on slayers.
MR Dragon Slaying, Rune of Cleaving: Combos with D3 wounds pretty well, overlaps a bit with Slayer Skill. Either HW or GW works. Hits on 3+ (usually), Wounds on a 2+, can stack wounds pretty well on most (excluding stuff like Chaos and HE dragons) behemoths.
Rune of Parrying, Rune of Striking: Defensive combo. I'd honestly prefer all out offense.