Approximate Catalog Value for Gutter Pairs?

I do not have a recent catalogue. I do however know that only the early issues command a considerable mark-up. The newer ones may still carry a small premium over twice the price of a stamp. I doubt that is what they are sold for. Unless they are 19780s or earlier, they are just two singles with a piece of selvedge in between. For the earliest ones, the mark-up is considerable, but not constant.

Later, Royal Mail purposely issued them for philatelists. In the 1970s and 1980s, counter sheets had two panes with a margin in between. Printer sheets later had several panes and were cut into smaller counter sheets. Then Royal mail decided to have also double pane sheets for their philatelic bureau, so collectors could order gutter pairs.