There is no need to take a taxi from Bangkok to Pattaya if you travel alone or 2 people. If you are 4 people a Taxi might be a better deal beucase the high price you need to get to the hotel in Pattaya.
The price from Ekami buss station to Pattaya is 128 THB and the busses are from early morning to 23.30 every day. There no need to book just drop by and take the next buss.
Some things to think about
On the night busses people trying to sleep please respect that by turning of mobile phone signals and talk quiet.
Eating is forbidden but snacks and liquids are ok.
There is a toilet on the buss but doing number 2 on the buss is not recommended.
The night buss stops on many places on the way to Pattaya.
Lonely girls on the buss are not all hookers and I have seen them call their Thai boyfriend crying for help when the foreigner jumps seat and start a conversation.
If you go off before Pattaya do not use all the seats as handles since the heads are resting on them.
Tell the driver before you leave Bangkok where you want to go off.
If you planning to sleep have your valuables tucked away since people have waked up with less money.
The trip will take from two hours to two and half.