Every single restaurant on the beachfront in Copa (and all the kiosks) will be organizing a party. Because everything in Brazil is done last minute, most won't even have finalized their pricing until 1 month before.
U$300p/p will cover a party at most hotels and any restaurants including food and alcohol.
Note that any restaurants at street level will not have a good view of the fireworks, so most guests will go to the beach for the fireworks and then return.
I would recommend Venga tapas bar, which should fall whithin your budget and has a great location. If you want to splurge and have a "once in a lifetime" you could consider the ballroom at the Copacabana Palace, complete with a gourmet dinner, but that will be in the vicinity of U$800-1000p/p