Known issue with the DX8 displaying voltage lower than it actually is. Typically 0.5 v discrepancy. Supposed to be fixed in a software update.
The batteries life expectancy can depend a lot on how it is cared for... We used to EXPECT NiCd/NiMh to need replacement every year regardless of care, but now if you care for even cheap ones properly you can get up to 12 years of service.
Actual cost of the DX8 is $300... (its always on "sale")