CREATE TABLE table_204_568 (id DECIMAL, "pennant" TEXT, "name" TEXT, "hull builder" TEXT, "ordered" TEXT, "laid down" TEXT, "launched" TEXT, "accepted into service" TEXT, "commissioned" TEXT, "est. building cost" TEXT, "fate" TEXT)

which ship had the highest estimated cost to build ?
SELECT "name" FROM table_204_568 ORDER BY "est. building cost" DESC LIMIT 1
SELECT "name" FROM "table_204_568" ORDER BY "est. building cost" DESC NULLS LAST LIMIT 1