Dashboard Widget - calculated columns based on a table? Looking for the best way to proceed.

OK, lets take this step by step.  You are asking for how to summarize that data, and for that you want to create a Summary Report.  Th wrinkle here is that you want the columns headings in a particular grouping relative to today.

I suggest making a new text field called [Close Date Bucket] and contriving to have it sort properly in alpha sequence.

For example:

IF(

[Close Date] < Today(), "  Past days",

[Close Date] <= Today() + Days(10), " 10 days",

[Close Date] <= Today() + Days(30), " 30 days",

[Close Date] <= Today() + Days(10), " 70 days",

[Close Date] <= Today() + Days(10), " 90 days", "Over 90 days")

Then make a summary report summarizing $$Value where the rows are by Market Class and enable column Grouping on [Date Bucket], 

Once you have that Summary Report perfected you can put it on a Dashboard or a Home Page.