A very inexpensive pen is the Hero 616, a clone of the Parker 51. You can often find them in 10 packs. It's an aerometric filler and holds a good amount of ink. It is a clip cap, though. They are certainly inexpensive enough to lose without much pain. The trim on some of a 10 pack may be misaligned, but the nibs seem to write well.
The Pelikan M200/M205 mentioned earlier is a nice pen. It's slightly on the smaller side but, with the piston fill, holds a good amount of ink. Pam Braun has good prices. Richard Binder tunes the nibs before shipment to ensure they write well, though at a bit higher price. That applies to all brands he carries and he carries several. The M215 is a fraction larger and a bit dressier. Interchangeable nibs may let you try different ones in the same pen body. The 400 will be a bit larger still. Richard's site has a pen size comparator that may be helpful.
I've bought from both Pam and Richard. My latest pen from Pam is my current favorite, though a bit above your price range. It is a Namiki Custom 823 Amber (demonstrator). It is a plunger filler, holds a ton of Private Reserve Chocolat ink, and writes very well. I paid $225+s&h, but am not sure what the current price is. If you're looking for something specific and you don't see it on her site, email to ask.
There are many pens that may fit your requirements, some new, some new old stock, and some used. For a wider look at pens, you may want to check out a pen show. The Triangle Pen Show is coming up in June.
There could be a bottle filled primary and a cartridge filled secondary with a backup cartridge just in case. (Leave the cartridge out and insert when needed to avoid drying the ink if you don't use it.) I even carried a backup to my 823 just in case, though I think I it was a Pelikan or a Conway Stewart. Lots of options.
