There's definitely a premium to be paid on a slabbed set over a raw set. Just look at the Legendary results. The difference in cards between these two lots are: Cobb/Green, Crawford/Bat,Elberfeld/Wash.Port
Mostly raw "set" of 516 (Grades 53% VG to VG/EX (with a few nicer)) sold for $27,500:
http://www.legendaryauctions.com/Lot...ntoryid=129500
All graded "set" of 519 (averaging 4.23, VG/EX-VG/EX+) sold for $65,700:
http://www.legendaryauctions.com/Lot...ntoryid=128991