20k sounds about right actually quite cheap.
This is purely about supply and demand and i have been in both positions.
once had to erect a huge sound barrier between industrial and residential sites about a quarter of a mile long and about 12 foot high in Long Eaton and could charge people to tip subsoil and basically got it done for free.
but i have also had to pay to have smaller sites filled for ground levels ( rarer but it does happen ).
depends on who is building what at the time ie banking for road works/flood plains etc or big holes being dug ie channel tunnel to name one