Checkpoint

This depends on how you are referencing or mapping these objects, but a high-level example (assuming you are indeed scripting your tests and not using keyword tests) would be to just create a few variables for all of the object's innerHTML, innerText, contentText or textContent property values. And example would be something like:

// xPath is referencing the 1st row (/tr), with a table header of 'Value Entered' (/th) and cell value that contains '£' (/td)
var deposit = Sys.Browser("*").Page("*").FindElement("//tbody/tr[1]/th[contains(text(), 'Value Entered')]/td[contains(text(), '£')]");
Log.Message("Deposit Amount = " + deposit.contentText);
// will print "Deposit Amount = £2,500" to the log

Again, this depends on how these objects are referenced in your project, but you can object spy the areas of the screenshot you provided to find how TestComplete is finding these objects and show you what properties, values, and methods you can use within your scripts.

An example would be something like the screenshot below, which is using name mapping, but you can see TC is attempting to find the object based on a generic xPath reference to a table cell, or <td> with 'Active' as a value, IE: //td[.='Active']. This wouldn't work bc there are several rows in the table which have cells containing a value of 'Active', which is why you may need to tweak your xPath or CSS reference similarly to how I did above:

OR add extra property selectors in your NameMapping file to help TC find a unique table cell (if you use name mapping).

Once you know how to find these objects and extract their values as variables in your test, the math/validation is pretty straight-forward. Something like:

// set your object's as variables
var deposit = Sys.Browser("*").Page("*").FindElement("//tbody/tr[1]/th[contains(text(), 'Value Entered')]/td[contains(text(), '£')]");
var payment = Sys.Browser("*").Page("*").FindElement("//tbody/tr[contains(text(), 'Total Estimated Cost of Transactions')]/td[contains(text(), '£')]");
var netAmount = Sys.Browser("*").Page("*").FindElement("//tbody/tr[contains(text(), 'Total Net Deposit Amount')]/td[contains(text(), '£')]");

// log for reference
Log.Message("Deposit Amount = " + deposit.contentText); 
Log.Message("Payment Amount = " + payment.contentText);
Log.Message("Net Amount = " + netAmount.contentText);

// remove symbol and convert to Int may be necessary
var depositInt = aqConvert.StrToInt(aqString.Replace(deposit.contentText, "£", ""));
var paymentInt = aqConvert.StrToInt(aqString.Replace(payment.contentText, "£", ""));
var netAmountInt = aqConvert.StrToInt(aqString.Replace(netAmount.contentText, "£", ""));

// verify math is correct - if deposit minus payment does not equal netAmount, throw an error
if (!equal((depositInt - paymentInt), netAmount)) {
  Log.Error("ERROR: deposit " + depositInt + " - payment " + paymentInt + " != netAmount " + netAmountInt);
}
else {
  Log.Checkpoint("PASS: math is fun");
}

if ((myvalueentered - myadviserpayment) = mytotalnetdeposit) then log.message("test passed") else log.message("test failed")

You can do the calculations in the snippet or you can have the values in variables and calculate on the variables. I would do the variables if I need those values again somewhere else.

It can key done in Keyword Testing, but it's a pain. You'll have more control in scripting the scenario you want. 

Though my example shows The property value "4782.4" does not equal "4782.4"!

You should use the Equality operator (==) in your if statement, e.g.

if (x == y)
  true;
else
  false;

Assume you're scripting in JavaScript, then the code should be,

if ((KeywordTests.Sample_Browser.Variables.Value_Entered - KeywordTests.Sample_Browser.Variables.Adviser_Payment) == KeywordTests.Sample_Browser.Variables.Net_Deposit) {
    Log.Message("Pass");
} else {
    Log.Message("Fail");
}

I don't think Code Snippet likes the if-statement as it keeps throwing an exception!

Marsha_R example is a lot easier to use.