Broker's fee
% = 1 - 0.05 * Broker relations skill level / e ^ (0.1 * faction standing + 0.04 * corporation standing)

Notice for the above formula :
e = approx 2.718
Both faction and corp standing goes from -10 to 10
everything at the maximum should give you a little under 0.2% fees.
The formula uses unmodified standings. The social skills won't help.

Transaction tax
1% paid by the seller.