What is the value of a 1928 B US 2 dollar bill?

Retail prices as of 10/2009:

$70 in average condition, $175 if nicer. Uncirculated ones sell for $1000 or more.

Red Printing

All 1928 U.S. $2 bills have red printing, indicating that they are a form of currency called United States Notes. U.S. Notes were very similar to the current Federal Reserve notes in that they weren't backed by gold or silver in the Treasury. U.S. Notes were issued up till the 1960s. Because there was no monetary difference between the 2 forms, U.S. Notes were phased out in favor of Federal Reserve Notes to save on printing costs.

Serial Numbers

A bill's serial number is a counter and security feature but rarely affects its value, nor does it help to readily ID a particular bill. The date, condition, and whether there's a small letter next to the date are much more important.

"In God We Trust"

That phrase wasn't put on $1 bills until 1957 and other denominations starting in the 1960s, so the fact that your bill doesn't have it is not a mistake.