The best one I know in this regard is the Platinum #3776 Century, mentioned above. It has a special sealing mechanism so that it can go for months without drying as long as you close the cap. On Amazon you can find Japanese sellers that sell it for around €60/€70. That probably doesn't quite fit "not very costly" in absolute terms, although it's actually quite a bargain for a pen with a gold nib (and a good one, IMO!) that would cost twice as much at a shop, and can last for many years.

I have heard that some Sailor pens also have a similar sealing mechanism, but I don't really know which. I have tried maybe a dozen fountain pens a bunch of inks, but I'm far from being an expert. However, if you go to the forums at http://www.fountainpennetwork.com/forum/ you will find real experts. You can ask for what you need and you will get answers for sure, it's an awesome community.