The cost might vary greatly, depending upon where the tree is to be located. In a small village, for example, the local parish council might be sympathetic to a 'd-i-y' approach, whereby you purchase your own tree (and the guards, stake and ties to go with it) and plant it in a spot of the council's choosing. (A plaque could be obtained from a good quality trophy shop, or similar).
So a sapling planted on parkland, or on the edge of common land, might cost you no more than £100. (Perhaps a tenner for the sapling, the same again for the the stake & guard, £30 quid for a nice plaque and a £50 donation to the council).
However putting a tree in a dedicated memorial forest could be far more expensive. 'Life for a Life' is a registered charity which manages such forests. Their services don't come cheap. The minimum donation to have a tree planted is £495, plus a minimum of £75 for a plaque:
http://www.lifeforalife.org.uk/
Chris