Here are a few affordable pens:
- Lamy Safari/Vista
- Kaweco Sport
- Pilot Metropolitan
- Parker Vector
- Tesco's if you can find them; $0.5 but write fairly well
The interesting part about fountain pens is customization: you can choose between different nib widths, type of paper and quality of ink. After some time you will notice significant difference if you go back to ballpoint/gel pens.
If you're interested, check out Fountain Pen Network: