Well, the job of PM is actually quite well paid by normal people standards, and comes with a decent pension. Plus the people that become PM are usually quite well educated or come from well paid fields. So you wouldn't expect ex-PMs to be poor, that in itself isn't a sign of corruption. And they aren't all corrupt. Gordon Brown for instance might not be everyone's cup of tea, but I don't think any one has accused him of being mercenary or corrupt.
I actually think that sort of blatant profiteering, such as the sort Sturgeons husband got up to (that she knew nothing about) is highly unusual and that's exactly why it made headlines. Likewise the bribes given to Gill and the (alleged) dodginess regarding 5 million pounds. Which is an enormous sum of money by UK politics standards. It puts camper vans to shame but that's not really the point it's unacceptable if it's a camper van or 5 million and I don't think "they are all in on it" really holds water.