'thicker is better' is not the rule. 100kg is not too thick though IMO, but if you can get between 40 and 90 then just do that. For corner bass traps in smaller rooms, chech out studio tips "corner chunk" solution...
http://forum.studiotips.com/ (can't find the exact link) It basically triangles of rockwool stacked up ontop of each other from floor to ceiling, or across ceiling, etc... cover with some porous, breathable, clothe, and you are in pretty good shape in terms of dollars per performance.