Inexpensive pens with flex nibs that are often mentioned are:
1. Waterman's: The pens from the 20s and 30s (52, 5, 7, etc.) can have some amazing nibs and can be found (not too easily) in working but not stellar condition for around $75. The 50s and 60s pens 5xx (and others?) often also have nice flex nibs and someone was selling them on the Green Board for $35 recently.
2. Parkers: Some Duofolds and Vacumatics occasionally show up with flex nibs, but expect to pay a premium for the pen
3. 50s german pens: Montblanc, Pelikan, Faber Castell and others made flex nibbed pens during this era and some of the less well-known brands show up around $50 sometimes.
4. Wahl: generally famous for their flex nibs. Pricey for the nicer pens, but there are some lower-end models that still have great nibs just not all the hype.
See Marketplace posts
here and
here for some options. At around $100, you can have your pick of the above brands. Lex Villines
lexvil@sbcglobal.net who is well-known on Pentrace often has flex-nib pens for sale on the green board.
I highly recommend the search for inexpensive flex-nib pens. I've been virtually doing it for a while now until the money comes back (where has it been?) and it's fun to keep an eye out for those things.