IF function: extending to read multiple cells/answers

Hi. Can you help. I use a standard IF formula to determine a holiday apartment rental cost i.e. I divide a weekly rental as a cost per day and this becomes multiplied by the no. of days of the rental.

I want to change my spreadsheet and the IF function so that is becomes (in words): IF the rental days are either 7, 14, or 21 days (from a list) the answer selected is the rental cost for the appropraite period i.e. if the rental days are 7 it picks up the cost (from a list) of, say, 100; if the rental days are 14 it picks up the cost of 200; if the rental days are 21 it picks up the cost of 300.
How can I enter the formula, using the IF function? Or, should I use some other functional formula?