Instead of using a $25 Li-Poly battery with a $10 charger, I was going to use one of these- https://hackaday.io/project/46436-3-cell-rc-battery-pack-made-with-18650-batteries What I was wondering Is how it would compare? It seems safer than a Li-Po, and cheaper, seeing how I have a bunch of those batteries sitting around, but performance wise, I'm clueless.
There's a few things wrong here:
1) A $10 charger is a bad idea, unless it's a trickle charger through the balance leads.
2) A $25 prismatic high-C lipo is a very different beast than 18650s, barring very specific ones (the LG HG2, Sony VTC5, etc) and very specific flight modes (minimum time-in-the-air of 30 minutes or longer). Most 18650s are sub-1C discharge ratings.
3) You need to spot-weld the tabs to the batteries. Do not solder them.
You also didn't say what kind of 18650s you already have.