It's an interesting point. I've been testing both V-Ray RT GPU beta and iray since they have been available and there do seem to be differences between the two that make an impact on what hardware is required.
Vray gives you the option to globally throttle the size of your textures so that your scene will fit onto the graphics card. With iray, if it doesn't fit onto the card it will only utilise the CPU, which i've found to be painfully slow (admittedly I haven't done as much testing with iray). On the flip side, you can use proxies with iray and I would hope you can do the same with vray, although they aren't currently supported in the GPU beta.
The cost of hardware will depend on your typical scene, and like Andrew said, ATI will be rolling out much more affordable 4GB cards next month or so (which wont work with iray) and then hopefully Nvidia will follow. Andrew, do you know how many processor cores they will have, whether they will have a gamer amount or a pro card amount?
With regards to the cost of the software, iray, whilst only available to subs holders, will mostly likely be included with Max 2012. So, if your not on subs it'll cost you £400 odd to get it now, which will be the cheapest way to get it and also means you will get Max 2012.
There are upgrade paths for vray, if you have it already, it'll be about £350.
So, all in all it looks like you could get either a vray or iray GPU solution for about £800 ($1250), split evenly between the software upgrade and a next gen ATI or Nvidia 4GB gamer card.
For a one off purchase I guess it will probably be worth it, but it's a tricky one.