I wouldn't focus on charging per word either, though this can be helpful once someone gets more specific with you about what they're looking for.
When you're giving a price, think about 1. how much research the piece requires 2. how much time it will take you to do this research 3. how much insight you'll need to have about the site's "personal" brand 4. how specific and original the work will be 5. how much experience you have writing pieces like this. 6. how many revisions you are willing to do.
1,000 words off the cuff is a lot different than 1,000 words that have been well-researched, edited, and revised.
My feeling is that you're asking how much you should charge for travel writing in general. For a 500 word travel blog post that required "light research", I would charge at least $100.
