It's even easier than you think. IT IS NOT NECESSARY TO BUY A 2ND ROLE!
When your talent points are zero...you can swap out any soul you have acquired. So, if you go to level 15 and you aren't happy and you want to change one out... save your 30g (which is not much but it is at level 15), pay to reset your talents for 1g, and then swap out the souls for the one's you want. Once you've spend points and saved it... you Soul selections are set in stone until you reset it again.
In Beta the Role prices were 30g, then ~5plantinum, and I didn't even bother to see what the last one would costs. While talent resets were 1g, 5g, 8g, etc... it was much cheaper to test this way to find the 'right' build.
Make sure you have acquired them first. After you first 3, you need to visit Sanctum or the Defaint equivalent and pick up the quests for the other Souls...one at a time.
However, as mentioned- it is very useful to purchase a 2nd Role for PVP and/or if you have an instance build... healer/tank etc.
Edited, Feb 21st 2011 9:49pm by Athalahelm
Edited, Feb 21st 2011 9:51pm by Athalahelm