TSQL UNION, But Get The Differance In Dollar Amount In The Result

GridView_1
Category Name SubCategory Name Amount
Construction Construction 2,877.00
Design Design 0.00
Soft Cost Inspection 0.00

GridView_2
Category Name SubCategory Name Amount
Construction Construction 2,800.00
Construction Contingency 300.00
Design Design 500.00
Soft Cost Inspection 980.00
Soft Cost Survey 145.00
Soft Cost Testing 720.00
Soft Cost Management 1000.00
Soft Cost Other Costs 10,000.00

QUESTION:
How could I UNION the two result sets together and where the Category Name and SubCategory Name are equal for GridView1 and GridView2, show the difference in the Amount column.

This is the result I am looking for.


Category Name SubCategory Name DIFFERANCE
Construction Construction 77.00 = (GridView1 - GridView2)
Construction Contingency 300.00
Design Design 500.00
Soft Cost Inspection 980.00
Soft Cost Survey 145.00
Soft Cost Testing 720.00
Soft Cost Management 1000.00
Soft Cost Other Costs 10,000.00

I am trying to do it on the back end. Though, if you have a clever way to produce a resulting gridview in C# with the results I need, I am all game!
This is what I have so far: (TSQL code)



SELECT

tblCategories.txtCategoryName,

tblSubCategories.txtSubCategoryName,

tblEstimatesLineItems.curEscAmount

FROM

tblEstimatesLineItems, tblCategories, tblSubCategories

WHERE

tblEstimatesLineItems.lngzEstimateId = 24 AND --@lngzEstimateId_Compare1

tblEstimatesLineItems.lngzCategoryId = tblCategories.idsCategoryId AND

tblEstimatesLineitems.lngzSubCategoryId = tblSubCategories.idsSubCategoryId

union all

SELECT

tblCategories.txtCategoryName,

tblSubCategories.txtSubCategoryName,

tblEstimatesLineItems.curEscAmount

FROM

tblEstimatesLineItems, tblCategories, tblSubCategories

WHERE

tblEstimatesLineItems.lngzEstimateId = 25 AND --@lngzEstimateId_Compare1

tblEstimatesLineItems.lngzCategoryId = tblCategories.idsCategoryId AND

tblEstimatesLineitems.lngzSubCategoryId = tblSubCategories.idsSubCategoryId