@juliomedina - I've used packages ranging from 25k to 100k and more. The larger packages deliver faster traffic, so it really depends on what your needs are. Getting traffic delivered faster doesn't always mean a better ranking. If Alexa sees that your site is getting a good flow of steady traffic they will rank you good. If they see you getting 1000 hits a day for 30 days and then you drop back to 100 a day they will rank you bad. It's better to get 100 a day over a longer period of time then these burst of hits here and there.
From what I've seen with Alexa is that they update every day. I check in the morning and then at night, never the same. My Alexa was around 500k when I started using this type of traffic. With this traffic and my organic traffic I'm now under 100k with a PR4 ranking, so I've been getting great results. That's the main reason I use this traffic, for ranking my urls. The better ranking the more I get from advertisers looking to place ads on my sites. Plus better ranking nets me more when I sell backlinks. Hope that helps!