The brake itself will range from a grand to $1250 or so. It pays to shop around. Installation extra, but if you're reasonably handy, the Jacobs is not a bad project to put in. It can be done at any time.
Don't know how much experience you have with a diesel, but it won't hold you back like a gasser on the same transmission. The brakes on the Ram are perfectly adequate for stopping you running around bareback or with moderate loads, but you'll also need them to hold you back on decent downgrades. The e-brake is a similar concept (only opposite) to a throttle plate on a gas engine. On the gasser, the resistance to the vacuum created when the throttle plate closes is what slows you down. With the e-brake, it's the backpressure from the throttle-like plate in the exhaust. It makes quite a difference. If I'm turning over 2500 in the lower gears and back of the go-pedal, it'll throw me into the shoulder belt, just like steppin' on the whoa pedal that I haven't stepped on yet. Brings to mind my mis-spent youth and a hard downshift on the old '66 Goat.
You have two options. I think they charge less for it if you order it as an option when new. If that price gives you pause, you can have it installed later, but it will cost considerably more. If the initial cost still gives you pause, you may want to try driving it without one and see how it handles for your needs/expectaions.
I wouldn't tow anything over 5k without one in the country I travel in, and find it comes in pretty handy for trimming up my speed running bareback, too. Generally, if it's not slick out, it's on.
Attention: TDR Forum Junkies 
