Cheapest way from Heathrow to London city center? Possibly close to marble arch?

One way from Heathrow underground to Victoria Station - £6.10 last time I check.

The cheapest way is to take the Underground from Heathrow on the Piccadilly line to Hyde Park Corner. That station is on the other side of Hyde Park from Marble Arch but you can get a bus to Marble Arch from outside the station. Or go one more station on the Piccadilly line to Green Park and change to the Jubilee Line for the train to Marble Arch . Thats probably the simplest.

The least expensive option is the Underground. But it will take an hour or more.

A faster option is the Heathrow Express train to Paddington Station.

Definitely the underground. Most likely a 1 day travel card. That gets you unlimited travel on the underground all day for £8.

ya exactly,underground is the best cheapest way..It takes around 50min.There are specific timings mentioned to travel say

05:02 and 00:18 Monday to Saturday and 05:46 to 23:54 Sunday and public holidays.