The entire Landsat archive of over 35 years worth of data for the whole globe collected by Landsat 1 to the current Landsat 7 is free. No charge, nothing to pay, absolutely gratis. Just search for the images you want, order them, total cost = $0, and USGS will email you a link where you can go and collect them. So go here for more info and ways to search and order data, but the best route in my opinion is GloVis. Now that’s what I call a data policy.
One word of caution, we already have a lot of Landsat data at BAS and you can search for what we hold here (BAS internal link only). So don’t bother duplicating effort and in an effort to manage our data properly, either ask me to order it for you or let me have the ftp link and I’ll sort it out and import it to your favourite geo-format at the same time.