Checked both FGW and NatRail and RailEurope for Ldn to M-in-M. The first two, with various dates from now, including tomorrow, both seem to run about 25GBP for one-way. RailEuope, with a Britrail pass (which may not apply to you), show it as cheap as $11 and as much as $100 without the pass. Don't know what your itinerary/plans might be but what you don't want to do is just walk up to the wicket at Paddington and buy the ticket for 'right now'; gets very expensive doing that. FGW is the 'carrier' while NatRail is kind of 'brokering' the tickets which obviously can push the prices up a bit. You'll want to play around with the various options on the sites; for example 'day-returns' can sometimes be cheaper than 'one-ways', different times of day/week can also affect the prices. A good way to do it is to buy the ticket online in advance and pick it up out of the automated machines in Paddington when you get there.