cheapest hmi
I have used two HMIs primarily and have dabbled in a few others.
I would say the first thing to look into is application, some are better for certain applications (and PLCs)...etc.
Wonderware's InTouch is a decent product. It's popularity makes it much easier to find support when needed. I think the scripting engine in Wonderware sucks. It hasn't changed (to VBa, like a few others) in many years. It can be clumbsy to use to times.
I have used GE's Cimplicity, but only with GE PLCs. I personally like it. It can be complicated to learn, but is quite powerful. It uses a VBa scripting engine so special components and ActiveX controls are very easily built and dropped in to the application. Sometimes I had a difficult time finding answers to tough problems.
Intellution makes a good HMI. I have not used it too much, but it worked well for what I was trying to accomplish. Pretty good support and versitile.
Parker makes a cheapo HMI, crude graphics, less flexibility, and very little support, but dirt cheap.
I have used the Automation Direct HMI demo. I liked it quite a bit. I have found good support from other products of theirs. Buti never developed a real application.
Hope this helps.