Possibly call one of the vendor sites you are looking at and ask for a quote
Go to the following web site to view iSeries model info. From there they have a link to request a quote.
I agree, you will want to contact a vendor to get an accurate price but you need to know how much disk space, memory, processing power (CPW) and other feature cards like ethernet connections, tape drive attachment cards, modem card, etc as each piece is separate. The systems run from a small model 520 or what ever they are calling the new Power Systems these days to the largest model 595 which is as large as the largest mainframe. Once you have configured the hardware, you will want to configure the base operating system and any additional features like Client Access, Backup and Recovery Software, Websphere, WebQuery, etc.
You will want to make sure you include all costs if you are comparing iSeries to other platforms including the cost of maintenance for both hardware and software, the cost of any third party software and of course the number of trained staff to run and maintain it.
Keep in mind that the Gartner reports still show the IBM i to be the lowest total cost of ownership in most major application areas like ERP when everything is factored in but the initial sticker shock will scare some people until they really see all the benefits and reliability and scalabilty of this wonderful platform.
Another factor to consider is that all the investment in the actual software is sustainable by the hardware even 20 years down the road becasue IBM keeps the hardware backward compatible even going from 32 bit to 64 bit (CICS to RISC) and into the future with 128 bit.
If you are just looking for some hardware prices, you can contact any IBM business partner and they will be able to quote for you. Alternatively, if you pretty much know what you are looking for, I've dealt with a company called Covenco in the past and found them to be very competitive. They also offer installation services as well.