The assumption built into your question that there may be a view premium that can be calculated as a percentage seems closer to me to the truth than the suggestion in most of the answers so far that there is a $5,000 to $10,000 amount. Further, I think the suggestion that the appraiser is the person to look for on this is almost precisely wrong. Given the rules they operate under, appraisers will almost universally undervalue subjective factors like views or a location backing to open space...or at the other end of the spectrum...negative features like a location on a busy street or with a lot of low flying airplanes. Buyers consistently place more positive and negative value on these factors than appraisers do...and the actual sales values of these houses reflect this. And while the appraiser doesn't necessarily include the full value of these as line items in the appraisal, the will look for comps that have similar features...and the increased/decreased value gets included in the appraisal that way.
My gut is that an outstanding view/location can easily add 10% to the value of a home... $25,000 for a $250,000 home and $100,000 for a $1,000,000 home. Particularly with very high end homes, the percentages may easily exceed this 10%.
The only way to pin this down would be to compare sale prices for houses on the view side of a street with those on the non-view side. In most cases, you'd have to go back 5 years or more to get enough sales to be useful, and then try to adjust for price changes over that period. One of the complicating factors will be that the houses on the view side of the street are often upgraded relative to the non-view side to begin with...and even if not...the people who own them probably put more money into them over time. So house quality will inevitably be conflated with view.