Laser Imagers- start out much more expensive, but data is more reliable and easier to work with (it's just a bunch of distance reading in polar coordinates. The data gives direct information that can be used. But you can't do as many things with the data as imaging.
Imaging- Cameras can be much much cheaper than lasers. Processing the data is much much more intensive hardware and software wise. Data interpretations are less reliable though you can do more with the data. If you try to get distance information with imaging, you probably need stereo, or even triple camera systems and those approach or exceed the cost of lasers.
If this is just hobby, my advice, transmit data to a PC and get the PC to do all the image processing (though I hate doing that just as much as the next person who likes self-contained robots). If you are a software engineer, I'd recommend transmitting all processing to a PC. I believe MATLAB is capable of doing such processing and if you can get it to work, it may help you save a lot of time. Going with lasers is just too expensive for a past time.
The cheapest laser I have found is the $2000 at Acroname (can also be found in a few other places). It only works to 4 meters though and is not an imager, just a line scanner. The next cheapeset lasers cost >$10,000 or so, but do imaging and work for km.
Maybe you can learn something about imaging and porting the processing to a PC by browsing around:
http://www.drrobot.com/