At the end of March, Romney had won 607 delegates and 4.1 million votes, meaning he spent roughly $18.50 a vote and $126,000 per delegate, CNN reported.
Among other things, the money was used to cover expenses such as hotels, food, office equipment, accounting services, travel, event consultants and online advertisers.
Romney is virtually assured of being anointed the party's presidential nominee during the Republican National Convention in August in Tampa. On Tuesday, he swept primaries in Connecticut, New York, Rhode Island, Delaware and Pennsylvania, home state to Rick Santorum, Romney's closest rival who suspended his campaign April 10.
"The Romney team was putting a lot of money out there," one Santorum adviser told CNN when the former U.S. senator left the race. "The budget was a factor."
At least two of Romney's rivals had a better votes-to-expenditure ratio than the former Massachusetts governor, CNN reported.
Former House Speaker Newt Gingrich spent $21 million through the end of March, picking up 141 delegates and 2.2 million votes -- or just under $10 a vote and about $150,000 per delegate.
Santorum spent $18.7 million on 264 delegates and 2.9 million votes, translating to just more than $6.50 per vote and about $71,000 for each delegate.
U.S. Rep. Ron Paul of Texas, however, spent about $35 million in the same time period, but received around 1.1 million votes and 72 delegates, or $32.40 per vote and about $485,500 a delegate.