The Home Depot sells fiberglass doors for as little as around $360, as of October 2015. The Masonite 9 Lite Painted Smooth Fiberglass Front Door comes in 38 colors and is pre-hung. Another option from The Home Depot is the Masonite Full Lite Primed Smooth Fiberglass pre-hung Front Door with Brickmold, which starts at around $450.
Lowes sells exterior doors for even less. Its ReliaBilt 6-Panel Right-Hand Inswing Steel Primed Pre-hung Entry Door sells for around $170, as of October 2015. This door is rust and corrosion treated and has an energy-saving core. It is ready to paint and ready to install.
Another affordable exterior door option is the Therma-Tru Benchmark Door Varissa, which has a 2-panel oval window and opens to the left. It has a smooth, paintable surface and is ready to hang. Its fiberglass surface is resistant to denting, splitting and cracking. The Varissa sells for around $350.
Another option at The Home Depot is the Feather River Preston Patina Full Lite Pre-hung Front Door. Selling for around $630, this door is bright white but can be painted to match any decor. It is made of fiberglass and built with polyurethane foam, providing energy-efficient insulation. This door comes with a limited lifetime warranty and its hinges are on the left.