Separator in data itself

Shell Programming and Scripting
1

Following is the CSV file, separated by ","

100,sunil,$1,000,mumbai
101,amit,$10,000,mumbai
102,sailesh,$10,000,00,mumbai

I want the following output:

100,sunil,$1000,mumbai
101,amit,$10000,mumbai
102,sailesh,$1000000,mumbai

Note: I know the number of fields in the file in advance. In this case it is 4.

Any help is highly appreciated.

Thanks,
SK

2 
sed "s/\([0-9]\),\([0-9]\)/\1\2/g" input_file.txt
3

using Perl:

perl -pi -e 's/(\d),(\d)/\1\2/g' filename