Currently there is no consumer HD video camera (under $3,000) that can perform well for chasing use. All of the consumer models use CMOS chips, which cannot capture lightning video without rolling shutter split-screen artifacting. Unfortunately many prosumer lines are switching to all-CMOS now in their newer models.
The least expensive camera suitable for all-purpose chase use is the 3CCD Sony HDR-FX1. That's the bare minimum you need for at least good lightning footage. However, even the FX1 lacks in low light performance compared to a standard-definition VX2100.
The cheapest HD camera that both uses CCDs and has good low light performance is the Sony XDCAM F350, a full-size shoulder-mount professional camera which will run you about $35,000 (body plus a cheap lens). That is the cheapest single HD camera model that can obtain a good image in nearly all chasing circumstances. If you've got more than $40,000 to spend, your options increase in number.
You could get away with spending $16,000 on two cameras (less than half the price of the F350) to get the job done. You could buy a Panasonic HVX-200 and a Sony XDCAM EX-1 for about $8,000 apeice. With its CCD chips and fast 100Mbps DVCPRO-HD codec, the Panasonic will get great lightning video - but it has small CCDs and is terrible in low light. The Sony EX-1 is much better in low light (for storm structure and tornadoes), but it is useless for lightning with its CMOS chips.
Unfortunately, CMOS is taking over due to its low cost and good performance. Lightning is too specialized of a subject for camera manufacturers to design for or care about. Even the $17,000 4K RED camera uses CMOS and therefore can't handle lightning (or anything with rapid flashes, like scenes with camera flashes or strobe lights).
FYI, here is a write-up I posted about lightning problems with HD cameras:
http://stormhighway.com/blog/may208a.shtml