Woodfall Primus saw its first and only reprint as a rare in Modern Masters 2013. This razed its price, down to about $3 a pop, after climbing past $10 a few months before.
After 3 long years, it seems that Woodfall Primus has removed all slack and is starting to ascend in price yet again. No card with a comparable effect has been printed in meantime: this is still a Modern staple and will remain so for year to come.
Foils are currently bottoming-out around $10.00. These will be $25.00 or more within a few months (likely popping after the non-foils). They are the better long-term bet.