Blind repairs are reasonable projects to undertake if you are mechanically inclined and have the right parts (and type of cord) for many standard cord operated blinds and shades. A fair sized pinch of patience also helps. If a replacement blind will cost less than $30, than it may not make sense to pay for a repair instead of simply purchasing an new blind.
Restringing, new cord locks or other mechanical repairs to custom shades or blinds, made by a major fabricator, are often far less exspensive than replacement costs. However, if the shade is an older shade one should consider whether this repair will solve the problem (such as worn or brocken cords) or whether the broken parts (brittle plastic) is just a symptom of more issues to come (on 10 plus year old verticals for instance).