I've had 3 experiences from BKK to Silom in the past month. Varied quite widely.
First experience was July 28th (there may have been some holiday as there were no highway tolls that day) - total was 350THB to the Meridien hotel at 15.00
Second experience was less pleasant - aggressive driver, asked for cash for tolls to pay (though he did give me all receipts - tolls were 75THB) - total incl. tolls was 500THB to Pullman Hotel G at 16.30.
First and second experience were metered taxis from the machine on -1 level
Third experience was with Grab - very pleasant overall - payment by card as opposed to cash - tolls again 75THB (driver paid). Total incl. tolls was 470THB to Pullman Hotel G at 17.00

I have two Grab fares from Meridien and Pullman G respectively (14.30 and 07.15) and they were 440THB (25THB toll - driver paid) and 455THB (75THB toll - driver paid) from late July/early August.

I will use Grab again today as I find the convenience much better and I don't want to have to deal with another aggressive taxi driver in cash fares and toll confusions. For me the fare differences between taxis and 'Grab Car' are minimal and the convenience and lack of stress make it a better overall experience.