Help with flight from Rio De Janeiro to Lima

Please can you help me in recommending how to obtain a cheap flight from Rio to Lima on 22 Februray, shortly after the carnival. We have been quoted £400 but would it be cheaper to book when we arrive in South America or would it be too risky as it might be busy at that time due to the carnival? we can be flexible with our dates by a day or two.

I am really stuck as to what to do - am on a low budget but then cannot afford to get stuck in Rio.....