Berlin is a great and cheap city to do on a weekend. You will definitely not need to spend 400-500 Euro for 5* accomodation... Most 5* hotels can be booked between 200-300 Euro max, especially on weekends.

I would not consider the Westin Grand to be a 5* hotel... Stayed there 2 weekends ago and got a great rate of 100 Euro per night.

My top 3 favorites are:
1) Regent Berlin (used to be the Four Seasons)
2) Ritz Carlton
3) Park Hyatt Berlin