The usual service (where the firm either provide pre-printed envelopes or the customer includes the word 'Freepost' when they write the address) costs �68.50 for the licence. (The company also have to provide an aditional �41.50 'up front' to pay for the first mail that they receive).
If the firm want to receive 1st class mail, they have to use pre-printed envelopes and they pay the usual first class mail price (32p up to 60g at present) for each item received. [Note: If the firm use envelopes with designs which meet specific guidelines, they can register under a scheme whereby the cost per envelope is reduced to 30.5p].
If they use 2nd Class pre-printed envelopes (or if the customer addresses the envelope themselves), they also pay the standard postage rates (23p for up to 60g, at present). [Again, note that if the company use pre-printed envelopes meeting specific guidelines, they can reduce the cost to 21.5p per item].
There is also a service where the company can have a special shortened Freepost address. e.g. simply 'Freepost BLOGGS'. This costs �166 for the licence (plus �55 'up front' in fees). All mail sent using this address, whether in pre-printed envelopes or written by the customer is sent 1st Class but there's a hefty surcharge with each item (up to 60g) costing 41p.
http://www.royalmail.com/portal/rm/content3?me diaId=600016&catId=5200009
and
http://www.royalmail.com/portal/rm/content3?me diaId=22100436&catId=5200009
Chris