Got Meogi's answer here working fine at my development site. Imported a fix to my production site and got an error:
a:5:{i:0;s:139:"Invalid method FirstScribe_CatalogOptionSortFix_Block_Product_View_Type_Configurable::_prepareOldPrice(Array ( (0) => (1) => 0 )
What might be wrong?
Found the problem. Earlier mod, which wasn't even active, hassled the outcome of the configurable.php Deleted all the files (not only xml from the etc/modules) and all is working again.