my wife and i have just finished 2 months down the west of south america and it's incredibly cheap.
to be honest for two of us we could have lived on £20 each per day without feeling like we were doing without we were also staying in a more expensive private room in hostels - dorms are much cheaper.
argentina and brazil are a little more expensive but in peru and especially bolivia you'll feel like you can't give your money away. travelling on your own is a bit more expensive than half the cost of a couple but i'd go for about £25-35 per day which will give you a little bit of luxury occasionally.
bit of a tip - don't go for the cheapest bus ticket you find. there's loads of operators and for litterally pennies more you get a decent, safe comfy bus. we bought cheap tickets and some of the busses were disaterous
I'm not sure if you.ve got things planned but we loves these:
la paz - Death road www.bside-adventures.com/
cusco - machu picchu lares trek
http://www.sastravelperu.com/
uyuni salt flats bolivia