The biomass source needs to be available locally in order to minimise transport costs and emissions. The price should be similar to that of fossil fuels.
The boiler is likely to have a slightly higher cost than a conventional boiler because of its somewhat larger size.
Grants
To be eligible to receive a low carbon buildings grant you must undertake a number of energy efficiency measures and you must use an accredited installer and an accredited product. The overall maximum grant for an automated wood pellet fed room heater/stove is £00 or 20% of the relevant eligible costs, whichever is the lower and for a wood fuelled boiler system the overall maximum grant is £1,500 or 30% of the relevant eligible costs, whichever is the lower.
Details are given at the low carbon programme website
http://www.lowcarbonbuildings.org.uk/