The EGO Z6 (ZT4204L) is a residential electric zero-turn aimed at quiet operation and low routine maintenance (no oil changes, fewer fluids). A representative new price is based on Ace Hardware pricing for the ZT4204L kit with four batteries and charger.
The key replace trigger is the out-of-warranty battery system: if multiple packs lose capacity or cell faults appear, battery replacement can be expensive and may exceed the 40% threshold quickly. Repair makes sense when issues are in the mower hardware (switches, wiring, blade control module) or when a single battery pack is affected and the rest test healthy.
Also factor usage pattern: frequent full-depth discharges, extreme heat/cold storage, and high mowing load can shorten battery life. If you're several years in and facing a large battery bill, replacing the whole platform may be more predictable than piecemeal packs.