Most conversions that I have looked at go for around $30,000. Here's on conversion outfit that offers some closer to Earth.

Here are some clips from their website blog with some details:

"Plug-In Supply Kit, PbA chemistry................................ $6,000 installed

* For 2004-2008 Toyota Prius
* 10-15 miles expected All-Electric Range
* 52 mph top speed (without using gas)
* Easy to use
* 360 lbs additional weight
* Upgraded rear suspension to maintain stock ride height
* 2-3 years battery pack life ($1000 to replace)
* 20 12v 20Ah BB PbA batteries, 256 VDC
* 6-8 hours charge time
* 110v charge supply (regular wall outlet)
* Access to spare tire
* No loss of cargo space
* Retain original HV battery
* Chassis upgradable to Lithium chemistry"

"Plug-In Supply Kit, LiFeP04 chemistry............................. $14,000 installed

* For 2004-2008 Toyota Prius
* 15-20 miles expected All-Electric Range
* 52 mph top speed (without using gas)
* Easy to use
* 150 lbs additional weight
* No suspension upgrade necessary
* 11+ years battery pack life
* 4-8 hours charge time
* 110v charge supply (regular wall outlet)
* Access to spare tire
* No loss of cargo space
* Retain original HV battery"

"Hybrids-Plus Kit, Lithium Ion chemistry....... $15,000 installed

* For 2004-2008 Toyota Prius
* 15 miles expected All-Electric Range
* 34 mph top speed (without using gas)
* Easy to use
* OEM level design
* No additional weight
* 110v charge supply (regular wall outlet)
* Access to spare tire
* No loss of cargo space
* Proven A123 Battery Pack
* Replaces original HV battery"

http://www.lusciousgarage.com/index.php/blog/

Luscious Garage On CNN:
http://www.cnn.com/video/#/video/tech/2008/09/23/obrien.hybrid.conversion.cnn

CNN mentions work in the area of adding solar panels to the car as well.

If it's for a Prius, Hymotion by A123 is good. A little over $10,000 including installation and a three year warranty.
https://www.a123systems.com/hymotion/home

oh noes a123
http://file.sunshinepress.org:54445/toyota-prius-a123-car-fire-investigation-report-2008.pdf

oh noes a123
http://file.sunshinepress.org:54445/toyota-prius-a123-car-fire-investigation-report-2008.pdf

Oh dear. That wasn't the new Hypomotion pack was it?

It was. Apparently it wasn't properly installed.

Summary
A Toyota Prius converted to a Plug-In Hybrid Vehicle configuration was destroyed by fire. The likely cause of the fire is improper assembly of bolted joints with electrical lugs. These joints became loose causing excessive heating which led to the rupture of individual battery cells which resulted in significant short circuiting of the battery ultimately resulting in the vehicle fire.
The batteries were designed so that a spacer should have been placed between the fuse blade and the electrical lug so that the plastic battery cover would not be in compression. Compressed plastic has a tendency to creep which would result in loss of clamp load on the fastener. Inspection shows that the brass washers that were intended to be the spacers were instead installed underneath the bolt head (between the bolt and the lug which served no purpose). Additionally, there was no locking mechanism (lock washer, safety wire, adhesive, etc.) that prevented the bolt from backing out of the nut. It is likely that
the bolted joint became loose over time and during regular vehicle operation which resulted in a high-resistance connection causing those components to increase in temperature.
A123 Systems’ design guidelines appear to be violated in several areas which may have contributed to the severity of this incident.