Help Needed: Working Out Price Bands..?

[NJ106]

Firstly, "hello" to everyone.. I've tried googling this etc but can't find any answers so here I am..

Ok, so here's the deal.. Trying to set up a database for the Gas Bill.. Say we get charged £1 per unit for the first 0-100units, then we'll be charged £0.75 for every unit after that (discounted price)

Any help on the formula..? I've tried countless methods but just can't get anywhere

Thanks