Actual cost, paid and plant rules shape business income computation under income tax definitions and special acquisition cases. 'Actual cost' is the assessee's cost reduced by amounts met by others, input tax or excise credit allowed, and subsidies, grants or reimbursements relatable to the asset. Cash payments above the prescribed limit are excluded, and proportionate subsidy allocation applies where the subsidy is not linked to a specific asset. Special rules govern amalgamation, demerger, gifts, inheritance, reacquisition, sale and lease back, scientific research assets, non-resident assets, corporatisation, and assets covered by earlier deductions or deemed income provisions. 'Paid' means actually paid or incurred under the relevant accounting method, and 'plant' includes ships, vehicles, books, scientific apparatus and surgical equipment, but excludes tea bushes, livestock, buildings, furniture and fittings.