Of course school alumni will tend to choose to contribute to their old school if it has given them a memorable experience that has led to their future success in life.
However, the economic ability of alumni to contribute is going to vary by school population - alumni who are successful lawyers, bankers, surgeons, top business people are likely to be able to contribute in more substantial amounts than alumni who are successful joiners, electricians, nurses, admin assistants.
So my understanding of school funding would suggest that, within any given local authority grammar schools will be less well funded than leafy comps with low %PP, who themselves will be less well funded than schools which take high proportions of children from deprived areas. It seems to me hard to argue that that is, a priori, unfair - though the huge variation between different local authorities seems to me to be unjustifiable.
However, the ability to raise additional funds from current parents and alumni is likely to be in reverse order - again in the same area, the grammar is likely to be able to raise more than the leafy comprehensive, which again is likely to be able to raise more than the school with high % of deprived children.
To what extent the funds from parents / alumni can compensate for lost revenue from central souces is unknown - schoolcuts suggests that Balcarras will lose £572,474, offset by c. £50,.000 per year, whereas Pates will lose £372,246, offset by c. £200,000 per year. That will change based on new government announcements, though, as Gloucestershire looks to be a significant net gainer from the announcement that the block grant will not be less than £4,800 in any area.