it is only an estimation but if you think about it what other data is there going to be? all the data you are pulling is the actual text posts and they go straight into your db. If you were pulling 1GB of data a month the worry would not be the bandwidth problem but the size of your database! I think matt said recently at sitepoint that a 1,300,000 post database was about 1.5GB or something which should give you an indication... no way will you pull anything near like a GB per month i am pretty certain of it.
About 1 more week i recon until i have another go with this, it has changed a lot since i first installed it on v 1.5