That's probably comparing an Open Return, valid on any train, buyable up to departure, versus a ticket limiting you to one plane, booked in advance - neglecting to mention the cost of City Centre - Airport travel by taxi, train or bus.
To do a fair test, I'll choose the same dates, and roughly the same departure times from City Centre stations and airports. [Outward date: 25 Aug - return: 01 Sep]
I just looked up on TravelSupermarket.com for the cheapest way of doing London [Any Airport] to Manchester. It said BMIBaby was the cheapest. Their 'Tiny Fare' was £10 each way, plus taxes at 48.10 return make a return fare cost £68.10. The flight leaves at 2050 and arrives 2150 on the outward, and 2100-2205 on the return.
For Virgin Trains, London [Euston] to Manchester [Piccadilly] on the 2105 the cheapest ticket avaliable is £37.00, and that's first class. On the return, the 2045 has the cheapest ticket of £12.50, totalling £49.50, including one half first class.
Even a Saver Return, valid evenings on both dates is cheaper, and weighs in at £57.10 - let's not forget this is even better vs. Air as it can be used on most trains that evening, with certain changes free, and other changes or refunds avaliable for an admin fee - it doesn't have to be booked in advance and further discounts are avaliable for Railcard holders.
Also, both offer a better choice of a refund should there be delays.
Oh, and BMIBaby charge a £4.75 credit card fee.
