Will never ... ,
The problem you're facing is that the Prius battery replacement industry isn't a commodity because it doesn't have to be done very often. You should be able to shop around and get better prices, however, there may not be enough business for Prius battery repairs to give you many options.
Remember that most cars today are ready for major overhaul or replacement at the 140K mile point. $5K is a lot cheaper than a replacement car and not much more than a complete head job + rebuilt transmission.