The answer depends on the volume of mail your franking machine will be handling because there are broadly three categories of franking machine, and you will find a few cheap machines in each category.
The cheapest machine in the low volume category (processing around 50 items of mail a day) costs around £1,000. These include the Pitney Bowes DM50, Mailstart 2 from Mailcoms, Neopost’s Autostamp 2, Frama’s Ecomail and the PersonalPost system courtesy of European Postal Services.
The cheapest medium volume machines handling 100-1000 items of mail a day also include models manufactured by Neopost, Pitney Bowes, Frama and FP and start from around £1,500.
The cheapest high volume machines start from around £9,000, with most of the big name manufacturers providing models in that range right up to £22,000.
You can find the cheapest franking machine in the UK selling at around £500, but this will be a used low volume model.