My understanding was that the 100Ah battle born batteries were $900 for 100Ah and maybe a little less if you gave them a call and asked for a discount.
They definitely can't compete on $/Ah at all with flooded lead acid, but compared to the higher end AGM batteries, they are starting to get closer if you consider $/usable Ah. They still don't make sense in a fixed application, but our light weight campers may be an example of an application where they are getting closer to making sense.
The DIY option definitely makes sense, but it is not a project for everyone.