The cheapest professional machines run into $50K unit prices. A RepRap will set you back under $1K, but it's not very useful at the moment.
I'm also assuming that you're talking about FDM, like the MakerBot/RepRap. If you're talking about a different process, well, then it's different.
There are also newer processes that are much more expensive. If you're checking out something like an Objet machine, or something like EBM, you're still going to be looking at an upper 5 figures to a mid 6.
I've compiled a list of most cheap printers (slightly out of date, I am going to add more soon): http://punkmanufacturing.com/wiki/
As far as usefulness, the open-source printers are not that different from the cheap commercial machines. A well-tuned RepRap should have comparable accuracy. The printing materials are a lot cheaper, as well as the parts (which you can print) and servicing costs. The commercial printers have better resolution, better materials and better software, at least at the moment, but I think that's going to change.
I'll just leave this here: http://open3dp.me.washington.edu/
Specifically: http://open3dp.me.washington.edu/2009/10/plaster-based-powde...
See how cheap that is? Crazy...
The Fab@Home guys are trying out various different materials, since their syringe-based extruder allows more flexibility than the RepRap.
http://fabathome.org/wiki/index.php/Fab%40Home:Materials
As far as cheap materials go, it doesn't get much cheaper than garbage plastic.
Or you could just go here (this is my old startup): http://marketplace.cloudfab.com/fab_facts (click through for every datasheet on every machine and material)
Most of the discussion I see about 3D printers ignores the significant amount of physical effort you have to put into a part if it will be used for anything (and depending on the method, just to see the part).
Now, they're totally right: the home printers have a significantly lower resolution than the professional ones. But unless you're making professional grade parts, you probably don't care, and they use the same materials. (I'm assuming you're making the comparison to other FDM machines.)
Sure they aren't great enough for everything yet, but there has to be things you can create even with the cheap bots that are good enough to be useful.
I'm excited to see what happens as the patents in this space expire. I'm expecting it to be similar to what happened with steam engines.