Upgrade to latest version

Does anyone know what the cost would be to upgrade from 1.3.4-sp3 to the latest version and is it a big job, I have very few modifications at this stage and figured it best to do it now rather than later.

Another thing is that I see there is a new basic skin and this isn’t available in my version (at least I can’t find it) it looks like it would be easier to modify than the others.

Anyone done an upgrade fro my version to latest had many problems