Dahab is cheaper for operators, agree. And for guests willing to dive for a couple of days, yes. But for 5-day packages or more, the price is virtually the same.
As you said before, Dahab is way smaller than Sharm. It is estimated that Dahab's capacity (in hotel beds) is 0.01 (1/100) of Sharm's. Dive operators in Dahab are around 60, while in Sharm they're only around 120.
In other words, Sharm is operating on masses, while Dahab is operating on individual service. The average operator in Dahab will have 20 guest in high season, and zero in low season. That's why wise operators can't play on prices.
Accommodation-wise, Dahab is cheaper if you choose camp-like stuff. Comparable accommodation to that of Sharm (Hilton, Meridien, etc...) is not cheaper. Actually 3* hotels in Dahab (and some 2* hotels) are more expensive than those of Sharm. And that's direct booking prices. Talk about all-inclusive and "lots" packages and you'll find Sharm way cheaper than Dahab.
However, one thing I like more in Sharm is that operators seem to be more cooperative with each other. In Dahab people are living in separate islands.