We took a black cab from heathrow to our hotel in central London, and the fare was 65 pounds. Is there an easy but more cost effective manor to return to heathrow extremely early on a Saturday?
It depends on what you mean by extremely early. The absolute cheapest way to get there is on the tube. The Piccadilly line goes directly to Heathrow, although the first tube of the day will depart just before 6ish (exact time depends on the station) and get you to Heathrow for 7ish. Depending from which zone you're starting in and whether or not you have an Oyster, the fare will be around £5. If you need to be there earlier, the Heathrow Connect train service from Paddington leaves at 4:42 am and gets to Heathrow Terminal 3 at 5:04 am. The Connect fare is just under £10 and you'll obviously have to get to Paddington as well.
Where is your hotel and what time do you need to be there?
For door-to-door service a private car service will be much cheaper than a taxi. Many people on this board recommend Just Airports They offer a discount for paying cash.
If you need to go before the underground starts you can use the night bus (N9) which follows a route similar to that of the Piccadilly Line. OK if you are close to this but not really if you have to connect to get to it.