Discount based on count

I have a table containing adverts. This is linked to a table containing advertisers, where I have created a formula that counts the number of adverts linked to an advertiser. I have a third table containing the percentage discount that will be applied on the advert’s price for the number of adverts that the advertiser has bought. For example:

  • 1 advert = 0% discount,
  • 2-5 adverts = 5% discount,
  • 6-9 adverts = etc, etc.

I cannot figure out how to use the column containing the count of adverts on the advertiser table and fetch the correct discount from the third table. Any ideas how to do this? I am trying to avoid hardcoding the discounting rules in lots of IF statements, but rather want these configurable on a separate table.