The reason a price has to be there rather than making it free based on achievements is so that future characters do not get these pretty powerful runes with only a minor effort, the runes do also offer significantly more power than the set bonus versions of them as they can be combined with any gear (including T6 gear) rather than restricted to T5 gear; therefore it is not completely unrealistic to expect to have to do some content to get them rather than just get a free upgrade. Personally I feel the price hits the middle ground between these factors.