SUM Count And ADD Expression With Multipliers

I'm trying to make an expression to calculate some fields in Excel 2007.
Trying to check fields up to a given number and give them a high multiplier, and add the rest of the numbers with a lower multiplier.

I have multiple fields with numbers in hundreds. I want to multiply the sum of this fields (up to 500) with 4. But the expression must also start the count from the first field, and when im hitting 500 as a sum I want the rest to be added with 2 as multiplier (the reason for the prioritizing of the fields is that they also have their own values/multipliers, but I'll skip that to avoid to much confusion here).

It's probably a lousy description. I'll give an example:

Field 1__Field 2__Field 3____Sum
100____300_____500______2800

How in earths name did I get that Sum you might ask.
((100 + 300 + 100) * 4) + (400 * 2) = 2800

400 is the number left from the third field wich I only want to multiply with 2 instead of 4.

Another example:

Field 1__Field 2__Field 3____Sum
300____600_____500______2000

((300 + 200 + 0) * 4) + (900 * 2) = 3800

900 is taken from the rest value of field 2 and 3.

If there any way of doing this "sum check and multiply, and then add the rest with a different multiplier"?