In what way is that HP tower an all-in-one form factor computer? You are not comparing like to like computers at all. You are not providing any information at all on what components, specs on the monitor or its capabilities. Is that monitor capable of 179º viewing??? I sincerely doubt it. You said you "built" an iMac with no upgrades? How do you do that????? You either buy an off the shelf iMac or you build one with upgrades. You don't have it both ways.
http://www.staples.com/MSI-20-All-in-One-Touch-PC/product-nr_866866?cmArea=SC3:CG71:DP4119:CL161746
Here is a 20” all in one computer/LCD combo—— $549 -—Touchscreen too