Public transport is cheap. Planes, trains, buses, minibuses and taxis from town to town. If you’re in a minibus or taxi, tell the driver you’ll tip him if he keeps the speed below 90/100 kmph! National speed limit is 90kph (120 on motorways).
Around Bkk try to use meter taxis with the meter on - it’ll be cheaper than the tuk-tuks (take a tuk-tuk once for the experience then use meter taxis).
Don’t let the drivers take you out of your way...they’ll try to take you to some store where they get commission.