Apply condition if the inserted value is within a range

I am working on Forminator on a site and getting stuck with calculations. What I need to achieve is that the client enters a free-field number, then depending on that value it is multiplied by a value, but the value changes depending on which bracket/range it sits in
ie.
101-200 * £7.80
201-350 * £6.00
351-650 * £5.507:46 pm
etc

That is when a value between 101 and 200 is inserted it will be multiplied by £7.80
and when the inserted value is between 201 and 350 it will be multiplied by £6.00

So if the client entered 143, it would be 143*7.8
or 305 would be 305*6