1) buy a new laptop (netbook, under the 250 USD)
2) install bitcoin core
3) do transactions
4) don't use this netbook to do others things
that's all.
price : 1 BTC.
It doesn't even have to be new. You can repurpose an older laptop (erase/wipe, partition, format).
From the savings, buy another router, and connect the bitcoin laptop behind it. Your netbook is now behind at least two routers, and two firewalls.
but it work flawless.
no need to trust 3rd party of a live CD ... for example.
and even a USB1.1 port can store the backup of bitcoin core.
While these are all good suggestions, none would technically create an
offline transaction, as they would all be technically online (although the setup would be safer then many setups that many people have).
For the truly paranoid (usually rightfully so as many people have made a lot of money stealing bitcoin from others) you would need to use a computer that has never touched the internet, nor will ever touch the internet in order to generate and store your private keys.