I am looking for a program that has an onscreen Heads-Up Display where I can write and edit notes that show up on my screen. The idea is to write notes like "tighten up CO, MP, EP ranges" if it's a really loose aggressive table, or the opposite if it's a tight passive table, or "3x BTN" if there's a fish in the blinds.

Playing 10+ tables sometimes the action goes really fast and I make a couple mistakes. Being able to jot down notes that show up onscreen seems like it would help out. In HM2/ PT4 you can take notes but you have to hover over the player HUD to view them, which is not what I'm looking for.

I don't know of any program that can do this, but would like to know if anyone knows of one. Also, how much would a program like this cost to develop?

I can suggest the following.
All your notes for players that you make in PokerStars client are stored in XML file from C:\Users\*your username*\AppData\Local\PokerStars
The name of that file is notes.*your PokerStars username*.xml

So the task is to find the software with functions for editing, viewing that XML file and searching in the lines of that file.

If you open XML file with text editor, it looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<notes version="1">
<labels>
<label id="0" color="30DBFF">Custom Label 1</label>
<label id="1" color="30FF97">Custom Label 2</label>
<label id="2" color="E1FF80">Custom Label 3</label>
<label id="3" color="FF9B30">Custom Label 4</label>
<label id="4" color="FF304E">Custom Label 5</label>
<label id="5" color="FF30D7">Custom Label 6</label>
<label id="6" color="303EFF">Custom Label 7</label>
<label id="7" color="1985FF">Custom Label 8</label>
</labels>
<note player="#douche14" label="0" update="1368542504"></note>
<note player="$AbRaO$ TT" label="6" update="1371841714"></note>
<note player="$MooneeyG$" label="0" update="1364053044">PLO SH 50</note>