Economics of scale will likely save you on cost by the time these are no longer covered by the warranty, as more and more vehicles are adding hybrid technology and requiring more batteries to be mass produced.
24 kWh Nissan Leaf (ours is only 0.4 kWh, 2% of the Leaf) batteries are $6k new, but will be getting refurbished and sold for $3k:
https://insideevs.com/nissan-introd...es-for-older-leaf-in-japan-from-new-4r-plant/
A start-up is aggressively pursuing manufacturing for $80 per kWh:
https://www.greentechmedia.com/arti...y-costs-below-80-per-kilowatt-hour#gs.zPO0rwc
Research shows the existing electric vehicle battery costs have dropped 60% over the last 10 years:
https://s3.amazonaws.com/files.technologyreview.com/p/pub/legacy/jan11_feature_electric_cars_p61.pdf
A government study showing the market for batteries and demand for mass production significantly increasing:
https://www.eia.gov/analysis/studies/electricity/batterystorage/pdf/battery_storage.pdf
The economies of scale have proven time and again that expensive technology gets much cheaper as it stagnates; LCD screens are a prime example, with costs being only 10% of what they were 15 years ago. It doesn't get much more stagnant then battery technology, and the demand is exploding as more devices make use of it.
The biggest risk is supply being bottlenecked by resources such as lithium and other rare earth minerals; the planet only has so much, and the more that is needed, the harder it is to find. Recycling will help to a large degree, since you can't just throw an old 18 cubic foot 1 ton battery in the trash and the only way to replace or dispose of it will be through a recycling center; however, that doesn't stop the other 90% of the small batteries from going right in the waste bin unless a significant refund is offered for them (e.g. a core charge for cell phone batteries when you recycle an old phone or swap batteries).
Personally, I am confident that my battery will be covered by some kind of warranty while it is still expensive, and will be relatively cheap to replace by the time the warranty expires; I'm expecting somewhere in the ball-park of the cost of two lead-acid 12-volt batteries.