There are strong indications that the cleanup of over 2,500 contaminated sites in Nigeria's oil producing areas would cost the federal government and other stakeholders over $50 billion.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/nigeria-cleanup-oil-producing-areas-cost-50-billion
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/all-africa
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/oil-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/nigeria
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ecology
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/unep
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/africa
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects