Location, location location! The places on the top are mostly saloons or “brasseries” (=bar/restaurant/brewery) that are very popular, thus the price is so high.
For instance, this is how “Le train bleu” looks like. Stunning, right?
My advice is to find the restaurant with the most perfect view of the city and the best quality coffee and just enjoy the one-time moment. You’ll worry about the price later 😉