I find Draylon's post a bit contradictory. Although students at Perse Prep do not take the Upper entrance exam children are "let go" ie. told they will not be offered a place at the Upper and therefore they are deselected without having to take an exam. I know of children who have been in this situation.
Not all children "pass" the entrance exams. Indeed there are 3 to 4 candidates for each place available at year 9 entry. So although possibly everyone who takes the entrance exam may score well 3/4 of them will not be offered places. There is no "pass mark".
As far as fees go - The Perse is pretty much one of the cheapest independents in the area and there are still many kids of Cambs academics in attendance at the school. It is for each individual family to decide whether their family income/outgoings means they can/cannot afford private education.
Their results were high prior to accepting girls and have maintained a similar level since accepting girls.
I am not sure what you mean about weeding out SEN. There are a good sized cohort of students who qualify for extra time in their exams for reasons such as dyslexia, discalculia, on the autistic spectrum and so on.
Yes the school is selective but knowing that, people generally don't put their kids up for the entrance exams unless there is a decent chance of getting in (ie. generally in the top sets /possibly set 2) at their prep or state primaries.
Also having had children at other independents and at The Perse I would say that The Perse is the most diverse in terms of race and "wealth" I have come across. There are number of fully funded places and part funded places by way of scholarships and bursaries. It is also does not have boarders.
Having said that I understand that Comberton is an excellent state option and have known people move to get into catchment area so I would encourage you to visit both and get a feel for both schools especially if you feel your child is academically able.
Good luck with whichever you decide is the best fit for your child.