In Access SQL you would use an IIf() function. The IIf() has the basic syntax:
IIf(condition, true action, false action) Either or both the true and false actions can be nested. So your statement would look like:
Select ... IIf(somefield<=100,20, IIf(somefield <= 250, 30, IIf(somefield <= 500, 50, IIf(somefield <= 1000, 75, 100)))) as NewValue,,,,
I abbreviated the conditions because when used in a query, the conditions in the IIf() are executed in order and as soon as a true condition is followed to the end, the IIf() is exited.
FYI - IIf stands for immediate If.