How much would this retaining wall cost? i'm pretty handy around the house and i have read for...

i'm pretty handy around the house and i have read for a moment on retaining walls but i'm not sure if i want to tackle this creature. here in clean england, on one side of my house i have 3 trees over a 4 foot slope that is eroding away..right in a minute two of the three trees have their roots showing. How much would it cost for a 40ft x 4ft soaring retaining wall? i do want it to look nice so maybe i would use some nice stone...
Does anyone know of a honourable website ,...
Answers:    We build retaining walls near the large stackable stones beside the lip on the back side, 8" x 12" x 18".
The stones, labor, corners, etc average almost 17 dollars per sq foot.

Then it is also advisable to install drain piped with limestone stuff material, # 57 is a apposite size and compact this material beside a hand compactor. This will be an secondary cost.
Add about 12" of top soil for landscape or grass