I recently had a sale on DP for a $97 product. I capped it at 50 sales and sold out within around 5 days. I tried to use Google Checkout because I wanted to keep the sale separate from everything I had going on in Paypal at the time...BIG mistake.
I ended up having to switch over to PP halfway through but couldn't edit the original post with the Goog Checkout button in it. Needless to say, it got to be kind of a headache pretty fast. But I got through it.
Paypal, E-Junkie, and social proof are all great advice. When you make terms, stick to them. I had many, Many, MANY DPers try to talk me into "helping them out" by saving a copy for them until they got paid or trying to make other "special deals" with me. I'd politely turn them down each time and even though a few were very (annoyingly) persistent, in the end it was always best that I stuck to my guns, ensuring the sale was fair to everyone. Also, no matter what, I know that avoiding bickering with people in my sales thread (no matter how tempting it is to call someone an idiot) worked in my favor as well. Show your professionalism at all times.
A 25 page ebook can be worth $1,000 or less than $1 - it all depends on the information it contains. Is the info immediately actionable? Is it concise? Does it adhere to your claims? It all comes down to whether or not the information is a good value for the consumer.
DP is like every other place in the world - give your customers value and you'll be successful.