cli and gui should both show how many bytes and packages are being saved (per jigdo download and overall) by using one or more --scan-dir or --scan-iso options and cache. (have to wait for the .template's to download first. summary output on completion.)

CURRENT EXAMPLE OUTPUT:
{{{
[jaf@nano:0j:222h:1e:0.51MB f8-20080501]$ time pyjigdo -f 'i386DVD*' --scan-iso=../f8-20080331/Fedora-Unity-20080331-8-i386-DVD.iso Fedora-Unity-20080501-8.jigdo
Fedora-Unity-20080501-8-i 100% |=========================| 191 MB 12:43
[1/2017] Packages/desktop-file-utils-0.13-3.fc8.i386.rpm exists and checksum matches.
[2/2017] lighttpd-1.4.19-4.fc8.i386.rpm 0 B 00:00
[2/2017] lighttpd-1.4.19- 100% |=========================| 370 kB 00:00
[3/2017] Packages/man-pages-ru-0.97-2.fc7.noarch.rpm exists and checksum matches.
[4/2017] Packages/xorg-x11-drv-cirrus-1.1.0-5.fc8.i386.rpm exists and checksum matches.
[5/2017] Packages/libgssglue-0.1-4.fc8.i386.rpm exists and checksum matches.
[6/2017] kde-i18n-Chinese-3.5.9-4.fc8.noarch.rpm exists and checksum matches.
[7/2017] policycoreutils-newrole-2.0.33-3.fc8.i386.rpm exists and checksum matches.
}}}

FUTURE EXAMPLE OUTPUT:
{{{
[jaf@nano:0j:222h:1e:0.51MB f8-20080501]$ time pyjigdo -f 'i386DVD*' --scan-iso=../f8-20080331/Fedora-Unity-20080331-8-i386-DVD.iso Fedora-Unity-20080501-8.jigdo
Fedora-Unity-20080501-8-i 100% |=========================| 191 MB 12:43
=== 1500 / 2017 Packages in template found in scanned data for a savings of 2012GB
[1/2017] Packages/desktop-file-utils-0.13-3.fc8.i386.rpm exists and checksum matches.
[2/2017] lighttpd-1.4.19-4.fc8.i386.rpm 0 B 00:00
[2/2017] lighttpd-1.4.19- 100% |=========================| 370 kB 00:00
[3/2017] Packages/man-pages-ru-0.97-2.fc7.noarch.rpm exists and checksum matches.
[4/2017] Packages/xorg-x11-drv-cirrus-1.1.0-5.fc8.i386.rpm exists and checksum matches.
[5/2017] Packages/libgssglue-0.1-4.fc8.i386.rpm exists and checksum matches.
}}}