Even some of the metered taxis will try to charge you a flat rate of about 200–300 baht to take you to Khao San Road, rather than use the meter (which would mean no more than an 80 baht fare from Silom, or 100-150 baht from Sukhumvit). The drivers will claim that Khao San Road is "too far away" for the meter, but that's not true; the fact is, they can get away with overcharging tourists, and if you don't take it, the next one down the street might. You should refuse to pay that amount and find an honest driver—there is no shortage of taxis in Bangkok. There is no point trying to haggle, as the meter is always cheaper.