Short answer. eBike battery replacement costs between $400 and $1200 for the pack plus $50 to $150 for installation. Rad Power OEM packs are $400 to $600, Aventon and Lectric run $450 to $800, and premium Bosch mid drive packs cost $700 to $1200.
Step by step
Get a load test first. Before spending on a new pack, get a proper capacity test. Many batteries labeled as dead actually have a bad BMS or a corroded contact.
Compare OEM and aftermarket. OEM packs match the original spec and keep app or firmware compatibility. Aftermarket packs are 20 to 40 percent cheaper but may not communicate with proprietary systems like VanMoof.
Ask about recycling. Any reputable mobile mechanic hauls away the old pack for proper lithium ion recycling. Never throw an eBike battery in the trash.
Consider brand and system. Orphaned brands (VanMoof, Juiced) may only have aftermarket options. Bosch and Shimano systems require OEM to work with the drive unit.