Once energy has been exported it flows on to the grid and will, in all likelihood, be used by the next door neighbour. However, there is no way of allocating that specific green energy to individual customers and so a smart meter wouldn't help.
Options for sharing green energy include setting up as a small supplier - there could be a new government incentive to communities where microgeneration is present.
Smart meters which record the energy bought from a supplier are installed by the supplier - and the supplier bears the cost. In the situation where energy is created via microgeneration, this would not be the case.
Smart meters are developing all the time, and costs will depend on the type of smart meter and also the quantity of meters purchased, so unfortunately we're unable to give an indication of price without more details.
www.electricity-guide.org.uk/home-power-generation.html
