How much should I charge for designing business adverts?

Hi, I'm new to the forum and I hope you can help me out.

I recently designed an advert for a friend's business, which will be published in a local business directory. Some companies submit their own ad to be included in the directory, while others tell the owner what they want in the ad and she pays a designer to create the ad.

The owner of the directory liked the ad I designed and asked me how much I charge for doing adverts. The ads can either be 1/4 page, 1/2 page or full page. A 1/4 page ad is 63mm x 86mm. A 1/2 page ad is 129mm x 86mm and a full page ad is 129mm x 178mm.

I'm not sure how much to ask for. I know it costs a company £35 to place a 1/4 page ad, £70 to place a half page ad and £140 to place a full page ad. That is how the owner makes money. The directory itself is free, so there is no fee for taking a copy.

With those figures in mind I was thinking of saying £20 for a 1/4 page ad, £35 for a 1/2 page ad and £65 for a full page ad. What do you think of those prices? Reasonable, too cheap, or too expensive?

It's a monthly publication so there's a good chance that any ads could be in the directory for a few months or even permanently. With this in mind, should I also ask for a small fee every time an ad I designed is featured in the directory? Maybe £5 per ad? Or is that an unreasonable request?

Any comments or suggestions would be greatly appreciated. If there are any magazine or publication owners on here, I'd also like to hear your thoughts.

Regards,

Joe.