You can convert from sparse matrix to indices and values of nonzeros entries, and then use sparse
to automatically obtain the sum in sparse form:
myCell = {sparse([0 1; 2 0]), sparse([3 0; 4 0])}; %// example
C = numel(myCell);
M = cell(1,C); %// preallocate
N = cell(1,C);
V = cell(1,C);
for c = 1:C
[m n v] = find(myCell{c}); %// rows, columns and values of nonzero entries
M{c} = m.';
N{c} = n.';
V{c} = v.';
end
result = sparse([M{:}],[N{:}],[V{:}])/C; %'// "sparse" sums over repeated indices