There are buses (A1/A2) departing from DMK to Chatuchat BTS/Mo Chit MRT that cost 30 THB per passenger. From there you can get to the Sofitel for 50 or 60 THB.

Alternatively you can take the city line from BKK to Makkasan for 35 THB and then MRT to Sukhumvit for 16 THB and hoof the final few hundred meters.

Those are the barebone cheap methods if that's what you're pursuing.

I wouldn't rely upon taxis being readily available at DMK. It can be hit and miss.