Obtener SALDO entre DOS TABLAS en UNA sentencia SQL

# Según consulta hecha en: http://foros.cristalab.com/obtener-saldo-entre-dos-tablas-en-una-sentencia-sql-php-mysql-t105748/#643660

# 1 - COBROS: SUMAR COLUMNA COBRO [RESULTADO ESPERADO: 330]
> SELECT SUM(p.cobro) AS cobro FROM tabla1;
> resultado obtenido: 330 [OK]

# 2 - VENTAS: MULTIPLICAR COLUMNAS CANTIDAD Y PRECIO_UNITARIO [RESULTADO ESPERADO: 830]
> SELECT SUM(precio_unitario * cantidad) AS venta FROM tabla2
> resultado obtenido: 830 [OK]

# 3.1 - SALDO: tabla2.venta - tabla1.cobro [RESULTADO ESPERADO: 500]
> SELECT SUM( t2.precio_unitario * t2.cantidad ) - t1.cobro AS saldo
> FROM  `tabla2` AS t2
> JOIN  `tabla1s` AS t1
> resultado obtenido: 2210.00 [ERROR]

# 3.2 - SALDO tabla2.venta - SUM(tabla2.cobro) [RESULTADO ESPERADO: 500]
> SELECT SUM( t2.precio_unitario * t2.cantidad ) - SUM(t1.cobro) AS saldo
> FROM  `tabla2` AS t2
> JOIN  `tabla1s` AS t1
> resultado obtenido: 840.00 [ERROR]

# 4 - ACERCAMIENTO
> SELECT SUM( r1.cobro )
> FROM (
>
> SELECT t1.cobro
> FROM table1 AS t1
> ) AS r1
> UNION (
>
> SELECT SUM( t2.cantidad * t2.precio_unitario ) AS venta
> FROM table2 AS t2
> )
> resultado obtenido: col1 = 330 | col2 = 830

# 5 - ERROR: #1222 - The used SELECT statements have a different NUMBER OF COLUMNS
> SELECT SUM(r1.cobro) FROM (
>    SELECT t1.cobro FROM table1 AS t1
> ) AS r1 UNION (
>    SELECT t2.cantidad, t2.precio_unitario FROM table2 AS t2
> )

# 6 - ERROR: #1054 - UNKNOWN COLUMN 'ventas' IN 'field list'
SELECT SUM(r1.cobro) - ventas FROM (
    SELECT t1.cobro FROM table1 AS t1
) AS r1 UNION (
    SELECT SUM(t2.cantidad * t2.precio_unitario) AS ventas FROM table2 AS t2
)

# 7 - SOLUCION [RESULTADO ESPERADO: 500]
> SELECT (t2.ventas - t1.cobros) AS saldo
> FROM (
>
> SELECT SUM( DISTINCT cobro ) AS cobros
> FROM table1
> ) AS t1
> CROSS JOIN (
>
> SELECT SUM( cantidad * precio_unitario ) AS ventas
> FROM table2
> ) AS t2
> resultado obtenido: 500 [OK]