The civil works account for Bt77.385 billion of the overall project cost of Bt101.112 billion.
The MRTA plans to call for bidding in October, with the results due to be announced next August and construction scheduled to commence late next year.
The route is expected to be operational in 2023 or 2024.
The South Purple Line is the extension of the Purple Line route from Bang Sue to Bang Yai, which commenced operation in August last year.
The extension is expected to witness a high number of passengers, averaging 477,098 per day.
The expected economic internal rate of return for the new route is 12.95 per cent.
MRTA acting governor Theeraphan Tachasirinugune said the agency was considering dividing the project into one elevated-route construction contract, and three or four tunnel-construction contracts.