In fact, the situation between best friends is very clear, so you don't have to worry too much. How much gift money to give for marriage depends on your actual situation. Generally, in your case, 500, 200, or 600 are all fine. As for what kind of wedding gift to give, you can take a look at the following suggestions.
Real-life ceramic sculptures: Real-life ceramic sculptures are very popular nowadays. You can go to a special gift shop and make a pair based on the real photos of your best friend and her husband. The shape is better to be funny and cute, and your friend will definitely be very touched.
Personalized customized jewelry: For example, name necklaces, personalized bracelets, engraved rings, heart-shaped hair accessories, brooches, etc., all are fine. The shape is better to be exquisite and special. There is nothing that women don't want.