Multi-Nested IF Sum

I hope some can help me with this one.

I'm having some trouble with calculating a value based on Tiers of spend and if it needs to be a sum or a general value.

I have vendor spend calculated on a Measure but I need to work out the actual tier value as well.

Supplier Number |    Unit     | Tier 1 Lower | Tier 1 Upper | Tier 1%

1                                        Spend      0                            375                       0.01

2                                        Fixed       0                             5                             1000

My Tiers go upto 7 but they are different on each row and the Fixed Unit always need to give the Value not a sum.