Experiencing this problem on a store. Prices are excluding tax and the store was upgraded from 1.7.
How I fixed this was.
update `catalog_product_index_price` set `final_price` = round(`final_price`,2);
update `catalog_product_index_price` set `price` = round(`price`,2);
update `catalog_product_index_price` set `min_price` = round(`min_price`,2);
update `catalog_product_index_price` set `max_price` = round(`max_price`,2);
update `catalog_product_entity_decimal` set `value` = round(`value`,2) where `attribute_id` = '75';
Reindex Product Prices to rebuild catalog_product_index_price