Bigger trams may be bought as Midland Metro vehicles struggle to cope with rush-hour demand on the current Snow Hill to Wolverhampton line.
West Midlands councillors will be asked to buy replace trams for the current service and planned network.
They will also be told expanding the tram network along Birmingham city centre streets, and through the Black County, could provide a 150% return.
There are plans to take the Metro to Wednesbury, Dudley and Brierley Hill.
The Birmingham city centre and Black Country routes have cleared public inquiries and have government backing.
Costs have risen to £281m, with construction inflation likely to take the final bill to £370m by the time the first trams are running on new lines in 2011.
'Particularly excellent news'
The bulk of the money will come from developers of major sites to be served by the 14.2 km (9 miles) of new routes.
Leaders of West Midlands councils have agreed to borrow the rest of the money.
Cllr Gary Clarke, West Midlands Passenger Transport Authority chairman, said: "What is particularly excellent news for us is that, when you add in developer and local contributions, the government would be getting a minimum double return on its investment: £1.50 of benefit for every 75p it invests.
"With those sort of figures, it becomes pretty difficult to argue that Midland Metro expansion does not represent good value for money."