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A toll booth owner wants to accept zero confirmation transactions (because waiting for confirmation would lead to long lines at the toll booth) and open the gate as soon as the transaction (with integrated payment ID) reaches the mempool.

Is there a way the toll booth operator can calculate the cost of a double spending attack against be Monero network based on current network hashrate?

The toll booth operator knows there are some tech savvy drivers who regularly use his road (that are smart enough to know how to attempt a double spend). However he wants to measure that impact against the lost revenue that will result if long lines cause some drivers to start taking alternative routes.

Accepting cash or credit cards is not an option because some of the users of his toll booth frequently never carry cash and value their privacy too much to use credit cards.

Logan
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It depends heavily on how much work has to be (provably) redone. If you act upon a transaction that has just hit the txpool, there is not work yet to be redone by a miner, so it's cheap. Same as for bitcoin. This assumes that the attacker is able to send a tx to your pool, while sending a different tx spending at least an identical input to mining pools. As monero and bitcoin use the same PoW system, attacks against either are the same.

Now, with your particular example, you probably have a way to determine who cheated, so you can either ban them from your toll gate in the future, or force them to use a queue where payments wait for one confirmation, which should in itself be a good incentive not to cheat :)

user36303
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You need 51% of the hashrate to pull any tricks with a blockchain. The hashrate (as of today) is around 20 MH/sec, at 10 USD per 1 Kh/sec, it would cost over 200,000 USD per day to perform one 51% attack.