I haven't checked the stats on this but generally most graduate jobs will see you earning between 18-25k but some may earn slightly more or slightly less. That goes for people how did sciences or humanities or whatever else.
However, generally anyone with a degree in a STEM subject will find employment much easier, so while a physics graduate may earn around the same as a history graduate, there are probably more physics graduate in jobs than those with a history degree.
As for working in marketing then it really depends on the degree and what the demand for it is and where you are willing to work. For example if you are only interested in a specific role and only in a specific area then you may well end up in a graduate job that doesn't care what the degree was in such as marketing (although marketing does prefer social sciences/business degrees just so you know), something like accounting would come to mind before marketing.
For example, if you did marine biology and want a job working with sharks in Cape Town, you'll likely not get anything. If you open your mind to anything from sharks in Cape Town to the National Trust in the far noth of Scotland, you'll likely get something.