How much do I need to spend on a good trackball mouse?

There is a wide price band. Budget thumb trackballs from brands like ProtoArc and Nulea sell around $28-$35 and carry large, mostly positive retailer rating pools, while Logitech's Ergo M575/M575S sits near $40 and the adjustable-tilt MX Ergo S and Kensington Expert run roughly $90. Higher price largely buys build quality, adjustable angles, and multi-device switching rather than dramatically better tracking.