been looking at the source for about an hour now, think i have it tracked down
Block.cpp line 188 starts
uint64_t transaction_fees = current_shares.count() * PAYMENT_SEND_FEE;
reward_to_share -= transaction_fees;
std::cout << "[rewards] pool fee: " << pool_fee << ", max transaction fees: " << transaction_fees << ", reward leftover: " << reward_to_share << std::endl;
in context after pool fee (plus a transaction fee for it) is subtracted (prior to this snippet) then a transaction fee is subtracted for every miner that has a share, before the miner rewards are calculated.
roughly running the math 5 percent of block reward is about 100 miner transaction fees. i doesn't look like the info is stored with the account (ie transaction fee taken from every block one), this make since assuming larger rewards and less division (so rewards sent more often) at the time it was coded.
there maybe more to this but this is the best explanation i got right now. it is late and i'm going to bed (hoping i didn't just add fuel to these flames).