Taxis are unmetered, and although fares mount up quickly, rates are fixed (your driver should have a rate card). Some cabbies may take you from St. John's to English Harbour and wait for a "reasonable" amount of time (about a half hour) while you look around, for about $50. You can always call a cab from the St. John's taxi stand.
St. John's taxi stand (268/462-5190; 268/460-5353 for 24-hr service).