The public spending watchdog revealed the costs for work carried out on the Borders railway, the Aberdeen bypass, the new Forth bridge, the Edinburgh-Glasgow Rail Improvement Programme and the upgrading of the M8 and other motorways in the central belt.
It also criticised the Scottish Government for failing to publicise the combined costs of the projects as well as being able to demonstrate the spending was affordable.
Auditor General for Scotland Caroline Gardner said the projects were key to the Government's aim of strengthening Scotland's economy and recovery, but stressed: "The estimated full public spending commitment for these five projects has not been reported until now and the forecast building costs for some have been incompletely or inconsistently reported.
"These projects will cost an estimated £3.8bn to build and will tie up about £7.5bn of public money over 30 years.
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"It is important for the Scottish government to demonstrate that this spending is affordable."
It is understood the Forth bridge is the only project where full costs had been reported, while auditors also looked at changes to the delivery of the Edinburgh-Glasgow rail improvements and the Borders line.
The report noted that 30-year costs were "commercially sensitive" in both cases.
Responding to the report, Transport Minister Keith Brown said: "No government since devolution has embarked on such an ambitious programme of investment to transform our transport infrastructure. We are making record investment in roads, railways and bridges that will improve links and boost economic recovery.
"We very much welcome this report which confirms that all five of the major projects underway in Scotland are being well managed with sound governance structures in place to manage the risks involved in what are technically challenging and complex projects.
"The centrepiece of our infrastructure investment is the Forth Replacement Crossing, as the biggest single transport project in a generation. We welcome Audit Scotland’s endorsement of the project’s sound management and recognition that construction remains on budget and on target.
"We also note the conclusion that the reasons for changing the scope of EGIP are 'clear and reasonable'. By revising the scope we will save taxpayers money through a reduction of £350 million in construction costs. Work on delivering the programme is well underway and already delivering benefits for passengers. The core Edinburgh Glasgow via Falkirk route will be fully electrified by December 2016."
Mr Brown added that the authority would take forward the recommendations made by the report to see where further improvements could be made with regards to their governance and risk management procedures.
(JP/MH)
