That suggests there is content other than just the numbers in the cells concerned, or that you have mixed data types in the column concerned. If it's the former, you may be able to resolve the issue by enclosing the mergefield in a formula field, thus:
{=«Rent» \# $,0.00 }
or:
{={ MERGEFIELD Rent } \# $,0.00}
The mixed data issue, too, is discussed in the Mailmerge Tips and Tricks sticky thread.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]