It is miles cheaper to make silage that way...and Miles greener....obviously there are considerations which may make it practical for you or not as have been mentioned above.
A farmer friend of mine recently made the switch, and I think he said the silage pit paid for itself in terms of saved net wrap and plastic costs alone in 3 years....but pretty sure he got a grant towards building the pit too.
I'll ask and get some ball park numbers next time I see him.