Comma Separated Variables might be the way to go. Kind of like
Id, Name, Price
1, Bob, 1.23
2, Fred, 2.34
3, Tom, 3.45
Depending on what the data is you can specify different enclosures, delimiters and terminators.
I don't know, but file rewrites every few seconds might be fastest with JSON. No particular reason for me to think this, just a hunch.
In any case, XML is more verbose though it can provide more control over the data.
And I'm wondering if using a database might just be faster and more efficient than files, though for that small amount probably not.
Sounds like testing might be in order before you commit.