Budget airlines like Southwest and Ryanair frequently make their passengers use stairs to board their planes and disembark. Some are even adding built-in stairs to their aircraft. Obviously it's to save money. But how much really? Simple Flying breaks down the extra fees that airports charge airlines to use jet bridges. From Simple Flying:
To put costs into perspective, consider London Gatwick airport. This charges a "departing passenger charge" of £14.95 ($20.46) per passenger. This is reduced by £3.48 ($4.76) if using a remote stand without a jet bridge. Daytime parking charges are also tripled if using a jet bridge stand (from the base rate of £10.68 ($14.62) per five minutes for the largest aircraft, for example)[…]
It is not just the cost of using the jet bridge that is a consideration here. The whole process and infrastructure come into play. As well as paying to use the bridge, airlines will require jet bridge operators and have staff located at the bridge area.