Totally depends on the publication and the location in the magazine, could be £3-400 for a full page for a less mainstream mag or £800-£1k for something more popular, up to £2k for an inside back cover or back cover.
Then if the Magazine has any exclusive deals with the people advertising you could be paying 4k a page. Obviously it can be cheaper as well depending on the time of year etc. You will pay more if it's a one off advert, many magazines only let you advertise if you agree to running for 6months+, although not always the case for more specialised publications.