MySQL Calculations using variables - mysql

Question

Why is this not working? I would like to get the result of:
(#sum_hours * #rate_hours) + (#sum_travel * #rate_travel) + (#sum_miles * #rate_miles)
The results of my sub queries are as follows:
#sum_hours = 5.00000, #sum_travel = 2.00000, #sum_miles = 0.00000
#rate_hours = 35.00000, #rate_travel = 35.00000, #rate_miles = NULL
#_rate_hours = CAST(IF(#rate_hours, #rate_hours, 0) AS DECIMAL(10, 5)) = 35.00000
#_rate_travel = CAST(IF(#rate_travel, #rate_travel, 0) AS DECIMAL(10, 5)) = 35.00000
#_rate_miles = CAST(IF(#rate_miles, #rate_miles, 0) AS DECIMAL(10, 5)) = 0.00000
So now that I have some values, I'm doing:
#total:= (#sum_hours * #_rate_hours)
I expect this to be 175, but it returns NULL. If I directly declare #sum_hours as 5
#sum_hours:= 5
I get 175, as expected. Am I doing something wrong? How can I get this to return 175, assuming I don't know that the values are supposed to be 5, 35 and 175?
#sum_hours is being cast as decimal, but with and without the cast it does not work.
Edit:
This is a stripped back version of the query, as the JOINs and subqueries aren't relevant, just the return details. All numeric fields are stored as FLOAT in the database. The values are confirmed in the result, but the calculation for #total does not work (returns null).
I have worked around this using COALESCE and some other bits, but I'm still curious as to why it would not calculate properly - or what I'm doing wrong.
SELECT
#sum_hours:= CAST(hours AS DECIMAL(10, 5)),
#sum_travel:= CAST(travel AS DECIMAL(10, 5)),
#sum_miles:= CAST(miles AS DECIMAL(10, 5)),
#rate_hours:= (SELECT ... returns 35),
#rate_travel:= (SELECT ... returns 35),
#rate_miles:= (SELECT ... returns NULL),
#_rate_hours = CAST(IF(#rate_hours, #rate_hours, 0) AS DECIMAL(10, 5)),
#_rate_travel = CAST(IF(#rate_travel, #rate_travel, 0) AS DECIMAL(10, 5)),
#_rate_miles = CAST(IF(#rate_miles, #rate_miles, 0) AS DECIMAL(10, 5)),
#total:= (#sum_hours * #_rate_hours) + (#sum_travel * #_rate_travel) + (#sum_miles * #_rate_miles)
FROM table
WHERE
id = '...'
GROUP BY id