It's still very early days for the aftermarket PHEV conversions, so early adopter prices are high. At least
3 companies are selling LiIon conversions around $10k, but a $3k DIY kit based on lead acid should soon be available from a group of hacker enthusiasts who are putting the kit together now.
May be cheaper just to wait for the first PHEV Prius direct from Toyota, it may be a factory option.