It's a 98GTP, neither turn signal will work. What is the most likely cause...bulb, turnsignal switch, fuse....etc.? And worst case, what is it going to cost to fix it.

Check the turn signal fuse located in the passenger side instrument fuse box. If it is good and 12 volts is present at the fuse socket then there is a high probability the multifunction switch has bad contacts. The multi function switch includes the turn signals and hazard warning. If 12 volts is missing at the turn signal fuse socket then a possible ignition switch assembly or wiring.

The switch and replacement can run around $500 at a dealer. However, it can be done DIY.