Another way to think about it is that if people were actually straight up buying votes on a regular basis the prices would likely be much higher. If politicians were admitting to themselves they were selling their vote, a large proportion of them would likely think about the value their vote have in a particular setting and price it accordingly.
Part of the reason it's cheap is because most of the time the politicians in question likely tell themselves they're not being bought, because they're taking donations for access etc. or other things that are sufficiently separate from a direct transfer of money that they too are pricing it based on access rather than on the value of a changed vote, as you suggest.
Part of the reason it's cheap is because most of the time the politicians in question likely tell themselves they're not being bought, because they're taking donations for access etc. or other things that are sufficiently separate from a direct transfer of money that they too are pricing it based on access rather than on the value of a changed vote, as you suggest.