There is no up to date formula about contract tax. And it’s obviously, no way related to market taxes.
What I find out for now:
Sale’s Tax
This is 1.0% of the base price with a minimum of 10,000 ISK and a maximum of 100,000,000 ISK. This is deducted at the successful completion of a contract. The skill Accounting decreases the sale’s tax by 0.1% per point, lowering it to 0.5% at level 5. The maximum sale’s tax is also modified accordingly (50M at level 5).The maximum amount is fixed, so you will never have to pay more than 7.5M + 50M = 57.5M ISK for a successful contract.
For courier contracts, these fees are all 1/10th of the amounts above and Sale’s Tax is omitted.
If someone grabs a courier contract and then fails it the deposit is returned to the issuer along with the collateral.
Deposit
This is 1.0% of the base price with a minimum of 100,000 ISK and a maximum of 10,000,000 ISK. If the contract is completed this amount is returned. If the contract expires or you delete it you will lose this deposit.
This one is wrong. Base value seams to be 2% and looks like it’s affected by standings and skills just like market broker fee.
Broker’s Fee
This is 0.4% of the base price with a minimum of 10,000 ISK and a maximum of 10,000,000 ISK. This is deducted when the contract is created and is non-refundable.
The Broker Relations skill lowers the broker’s fee by 0.02 % per level so at Broker Relations level 5 you will have to pay 0.3% in Broker’s Fee. This skill also influences the maximum deposit and lowers that accordingly, so at level 5 you have 7,500,000 ISK maximum broker’s fee.