The effect is different depending on the number of decks being dealt. On a single deck, it costs the basic strategy player .13% to eliminate doubling against a 9 and another .13% for no soft doubling, for a total effect of -.26%. With an "infinite deck," which is a theoretical measure used to quantify rules values and address other blackjack questions, it costs .08% for each, or -.16% -- a lower impact because basic strategy calls for doubling less as the number of decks being dealt increases.
The Wendover casinos -- Montego Bay, Red Garter, Peppermill, Rainbow, and Wendover Nugget -- all deal single- and 6-deck games, so the house edge is increased by .26% on the single decks and by .18% on the shoes. This sets the overall house advantage at .44% for its single decks and .94% for 6 decks (notice that even though the rule costs more on a single deck, it's still the much better game, with a house edge less than half that of 6 decks).
Monetarily, restricting doubling to 10 and 11 costs players an extra 18¢ against 6 decks and 26¢ against single decks for every $100 wagered. That's only about 25¢-$1.50 per hour at minimum bets, which is trivial. However, $25 bettors are penalized in the $10-per-hour range, and the rule adds up to a lot of extra money for the casinos over time. Or at least they think it does. What's interesting is that imposing the doubling restriction may wind up costing the house money, as poor players no longer have the option of making incorrect double downs, especially with soft totals.
Bottom line, since you should know how to play basic strategy perfectly, this rule is a negative for players -- and it's not the only one the Wendover casinos have made recently. We've heard that video poker schedules have also been downgraded at all the casinos -- another example of how a lack of competition results in an inferior product for gambling consumers.
As an aside, note that a discussion like this shines a light on how bad the 6-5 payoff on naturals is, as that rule, at a cost of about 1.4%, is about seven times worse than this double-down restriction.
