Actually, PostBooks IS free. It's open source software distributed under the CPAL license. Users may download it, install it, run it, and even modify it, all free.
The software is client/server, meaning that to run it you need both the client software and a PostgreSQL database (included in the PostBooks installer package). For users who want to try out the client without running their own database, xTuple offers an online demo account. This is the free "demo" you are referring to, and generally it is offered only for a limited time, to save space on xTuple's server. When the demo expires, there is no restriction on using the software client and database on your own computer or network.
The xTuple website lists prices for support contracts and software licenses for xTuple ERP Standard Edition and OpenMFG Edition. These are versions of the software that offer all the features of PostBooks, plus additional features for large-scale distribution and manufacturing operations.
explains how to download the software and, particularly, the difference between the all-in-one PostBooks installer and the different components available for advanced users who want to set things up their own way.
Like all xTuple products, it runs equally well on Windows, Linux, Mac and mobile – and is fully internationalized (multi-currency, support for multiple tax structures and multilingual translation packs maintained by our global community). PostBooks is licensed under your choice of CPAL, the Open Source Initiative (OSI)-certified Common Public Attribution License, or xTuple’s commercial license.
PostBooks is a full-featured, fully-integrated business management system, the core of the award winning xTuple ERP Suite. Built with the open source PostgreSQL database and the open source Qt framework for C++, it provides the ultimate in power and flexibility for a range of businesses and industries of any size.