Best mid-range pick: Leaderfins
Leaderfins make 100% carbon freediving fins at relatively affordable prices. Pictured here in basic black, they have lots of other colors to choose from. Their long carbon blades provide plenty of power. They are heavier than higher-end carbon fins like Alchemy's.
Note: avoid what they call "Carbon Fiber Bi-Fins", which actually use a mix of carbon and fiberglass in the blades, and look for what they call "100% Carbon Bi-Fins", which use blades made only from carbon.
Note: factor in their shipping when checking your budget - it is currently about $60.