Hi,
Looking at TFL website, I'm advised to get to St Pancras international station and take the East Midlands Trains to Luton Rail Station.
(I assume that Luton Rail Station is a walking distance from the airport, right?)

How could I tell what the total cost of the journey is?

I'm trying to figure out how much more expensive it'd be compared to easybus.

Thank you.

It is about a 2 mile walk from the station to the airport - however the Luton airport site http://www.london-luton.co.uk says that there is a shuttle bus between the two.

Take Thameslink railway from 6 stations in central London (just one of these is St Pancras) to Luton Parkway station and catch a shuttle. There may he a way to buy the rail ticket that includes the shuttle, else about 2 pounds more.You can't walk, the access road goes thru a tunnel.

For the sensible route - the train - ticketing and actual travelling aren't quite as straightforward as you might imagine.

You buy, online at a machine or at any ticket desk, a ticket to Luton Airport (LUA). Do NOT buy a ticket to Luton (LUT) or Luton Airport Parkway (LTN)

You get off the train at Luton Airport Parkway (the station before Luton). Outside the station, there's a frequent connecting bus to the airport. Your ticket is checked getting onto the bus, and if it's for LTN or LUT, you have to buy a separate ticket, which can be another hassle with luggage amid the crowds.

The train (and the connecting bus) can be crowded and you may have to stand. BUT it's reliable. Easybus has an awful reputation for reliability, and - even if its own operations were reliable - has to compete with serious traffic congestion.