$3.7 billion.

That's what the city expects to spend on building an elevated commuter train connecting East Kapolei to Ala Moana.

The number has been memorized by both supporters and opponents of the city's proposed mass-transit system.

Supporters view it as money that will be well spent and argue that if the system isn't built now, it will cost much more in the future.

Opponents say the cost is greater than the benefit and that less costly alternatives could do more to reduce traffic.

But unless you're Bill Gates, it may be difficult to put $3.7 billion dollars in perspective. So here is a little primer on what $3.7 billion is.

First, $700 million is expected to come from the federal government. That leaves local taxpayers with a $3 billion bill to pay over a 15-year period.

By some measures $3 billion may seem small.

For example, $3 billion is just 4.6 percent of the $65 billion value of all goods and services produced in the state each year. It's also one-quarter of the $12 billion spent by Hawai'i visitors annually.

By other measures, it can seem huge.

That $3 billion could buy nearly 140,000 ultra-fuel-efficient Toyota Prius autos — one Prius for every four or so licensed drivers on O'ahu.

Or $3 billion could provide free gasoline to all O'ahu drivers for more than 2 1/2 years (at today's prices).

That $3 billion is nearly twice the cost of the state's largest public works project to date, H-3 Freeway. The H-3, which connects Pearl Harbor and the Marine base at Kane'ohe Bay, was completed in December 1997 at a cost of $1.3 billion, or about $1.6 billion in today's dollars.

In physical terms, 3 billion $1 bills laid end to end could go around the Earth nearly 12 times. Three billion $1 dollar bills placed in a stack would stand about 231 miles high and weigh 3,300 tons. That's the weight of nearly 15 Statues of Liberty.

It would take the average Hawai'i income-earner, who makes $39,239 a year, about 76,000 years to amass that $3 billion.

But again, to Bill Gates it may not seem like much.

That $3 billion would buy just 1.2 percent of the total stock of global software giant Microsoft Corp.

Three billion. Big or small? It's your call.