Watch out for the combination of "cheap" and "compost"... Picloram and
Clorpyralid are a couple of broadleaf herbicides that may survive composting...
they've gotten into compost before, with devastating effects on gardens: e.g.:
http://cahenews.wsu.edu/RELEASES/2001/01035.htm
http://www.cityofseattle.net/util/clopyralid/
This is a special problem with compost made from yard wastes. A friend whose
garden was affected by it in Pullman is still seeing residual effects several
years later.

Peas are good test cases to use for checking to see if a compost
is contaminated. See: http://css.wsu.edu/compost/bioassay.htm

Kay Lancaster