A link to the current site would help us see what you have - and the degree of customization...
You are asking for an upgrade - but say you just want the install guide with files??
The install guide and downloads are available from links at the top of this page ('Download' and 'Documentation'.)
If you are running CPG 1.3.x, you will have to do a 2 step upgrade:
Upgrading from version cpg1.0, cpg1.1, cpg1.2.x or cpg1.3.x to cpg1.5.x
Support for a direct upgrade from cpg1.0, cpg1.1, cpg1.2.x or cpg1.3.x to cpg1.5.x has been dropped - if you still have such an ancient version running, you will have to upgrade in a two-step-process (from your version to cpg1.4.x and then on to cpg1.5.x)
(Note the link to the 1.4 documentation appears to be broken - we'll get that fixed...)
There were significant changes from 1.3 to 1.4 in the theme engine and move from mods to plugins - the degree of customization of your site will certainly influence the cost if you want to pay for the upgrade.
A new installation is quite easy following the docs if you prefer to go that way.
I will be traveling for the next week - but will be available following that if you need assistance and can provide more details.
I can provide a price estimate via PM once I can see more of the site (if it is even working... CPG 1.4 has issues today as ISPs upgrade levels of PHP and MySQL... I haven't seen 1.3 run in quite a while.)