Yes very expensive.
I was lucky enough to do a speeder run 2 years ago.
$500 is a lot, but the price does include a lot more then just your typical speeder trip. You get to ride a the train to Silverton and take a speeder back. You also get a full mow uniform that the mow crews wear. This includes shirt, overalls, gloves, protective glasses, and a hardhat. Fun to wear around the house and pretend your a real train guy
And it's not like you go down the line for a few miles. You run the entire line up to the speeder shack north of the Durango depot. The crew van will take you to the depot.
You can stop along the right of way if you want to take photos, etc... The speeder guy that goes along with you pretty much lets you go where you want. I have some great photos that you really can't get any other way from that trip.
On my trip we followed 473 on a test run after a recent fix up. I was able to get some incredible videos of the loco running light. We even dowsed a small fire along the way.
Some great memories and it was sad when the trip ended. Not to downplay Nevada Northern, but the scenery along the Durango & Silverton is just a tad more beautiful, no? And to me a lot more interesting of a line. But is it worth $1000 in a steam loco? Well that's up to whoever wants to spend that much...the speeder trip was expensive enough.
I've heard of some people taking the cabride up and a speeder trip down. $1500 round trip