This study about Reaching the Travel Audience produces some interesting statistics in favor of destination websites that are certainly positive and support the fact that DMO are well advised by building effective websites.
The conversion rate comparison to online travel sites, however, is the proverbial "apples to oranges" comparison. What’s measured on Expedia, Travelocity etc. is the actual product purchase. On the DMO sites it is future stay after having visited the destination website for information. Two entirely different activities. This is like comparing the return rate of a direct mail campaign to the purchase rate, which naturally is much lower.
Those destinations that are selling travel services on their own sites directly are probably showing rates of conversion from site visit to online purchase similar to those of online travel agencies.