The price of biomass fuels like logs or pellets depends on your location, and changes over time depending on local demand and levels of production. At the moment, we're producing many more pellets in the UK than we're using - so we're actually exporting to other countries in Europe.
Typical prices for pellets are between 4 and 5 pence per kWh (kilowatt-hour, the standard unit of energy prices), whereas wood chip at a 30% moisture content is between 2 and 3 pence per kWh. Logs will be cheap when bought in bulk in rural areas.
Bought in bulk, wood pellets can work out at about 5 pence per kWh if bought as a pallet of around 100 x 10kg bags (1 tonne). If you have a large hopper and can buy about 3 tonnes of pellets at once, then the cost would drop to perhaps 4.5p per kWh. The exact price will depend where you live - you can use some of the links below to get quotes.
One tonne of wood pellets has an energy content of about 4800kWh, so a cost of 5p per kWh equates to about £240 per tonne and 4.5p per kWh equates to about £215 per tonne.
This price per kWh is for the energy content of the fuel, so how efficiently you burn the fuel (plus how good your heating controls are and how well-insulated your house is) will then be important in how much of that energy becomes useful heat in your house. However, as modern pellet boiler is almost 90% efficient, this is similar to a modern oil or gas boiler - so losses in each case should be comparable for the same house. Burning logs or oil in an older or more basic appliance (e.g. a range or stove) will not be as efficient, so your effective heating costs will actually be much higher, and this needs to be taken into account when comparing options.
Biomass fuels compare favourably to other options: heating oil at about 6p per kWh, LPG at about 8p per kWh, and mains gas (where available) at about 4.5 to 5 pence per kWh (the kWh, kilowatt-hour, is the standard unit of energy prices you'll see quoted on your bills).
For comparison, mains gas prices work out to 4 to 5 pence per kWh for typical households (including standing charges etc.) whereas heating oil and LPG are typically 6 to 8 pence per kWh (note that the LPG price is for bulk deliveries, e.g. 1000 litres, whereas filling smaller propane bottles can be significantly more expensive).
Pellets should be pure heartwood, so you should not really see dark flecks from bark (and especially not coloured flecks from paint!) in good quality pellets. The highest standard is the EN+ A1. See for example the UK Pellet Council, the ENplus website, and the HETAS fuel suppliers list.