[bunderbunder]

If you're just looking to dip a toe in the water, Pilot Varsity is a disposable fountain pen that can be had for $2-3 in the US, though it's usually sold in multi packs. IMO, it's a decent writer for the price (and the only disposable I've tried and would recommend), and lasts long enough to see whether you'll get comfortable with the way a fountain pen behaves.

Refillable pens can be had for not a whole lot more, but the ink reservoir will be smaller, so, even though they usually come with a free cartridge or two, you may end up wanting to buy a few extra cartridges or a bottle of ink in order to get enough writing time to really get a feel for the pen.