Typical round here (hilly West/North Yorkshire, Lancashire, North Derbyshire) is 20 metres per km (~106 ft per mile). Very hilly rides are more like 25 metres per km (~126 ft per mile). I did one killer that was about 28 metres per km (148 ft per mile). That is for rides of 40+ km (25+ miles), starting and finishing in the same place.
I could choose short hilly routes with numbers MUCH higher than those by simply going up a steep climb and coming straight back down again. For example, Pudsey Rd/Shore New Rd from Cornholme up to The Long Causeway. That climbs 200 metres in 1.93 km (656 ft in 1.2 miles) which is a horrid 52 metres per km (273 ft per mile) up then down. Obviously, any practical ride will come in less than that but if I were crazy enough to ride there, do multiple hill repeats, and then ride home I would come up with some huge number.
I did a 179 km (111 mile) ride yesterday the route for which was chosen to be 'flattish'. That came out to be 1,800 metres (5,905 ft) so ~10 metres per km or 53 ft per mile. I didn't start and finish at the same location, but there was only 7 metres in it so not a significant factor.
The Manchester 100 sportive in Cheshire is often described as 'pan flat' but even that has about 1,000 metres of ascent in 161 kms (~6 metres per km) or 3281 ft in 100 miles (~33 ft per mile).