Advice on the best inexpensive fountain pen that uses Standard/International cartridges?

So, I was at the Montblanc shop in the airport and they were offering 20% off MSRP on everything. I wish I had enough to afford the new MontBlanc Heritage in Coral, but I digress. Anyway, I bought a 60mL of Mystery black and Burgundy red. They're not my top choices for red and black, but they are both attractive and write well in my Lamy Safari.

Anyway, I'm checking out and telling the clerk that I was looking for Oyster Grey, but they didn't carry it (they do have Permanent Grey for $20, but in a smaller bottle size). So she offers me some of the standard ink cartridge samples for free! I get the set of all five color options and it turns out they're "out of date" and she gave me three sets. Fifteen cartridges total!

Basically, I'd like an inexpensive pen that will allow me to use these cartridges. It would be awesome if I could find a flex nib or type of flat nib calligraphy style pen for art projects and goofing with. Your input is much appreciated! My price range is $10-$20. I've looked at Jinhaos and they look okay, but I'm looking for a doodler/calligraphy and lettering practice pen, not a pen that focuses on looks, and comes short on quality.