Don't get me wrong, Sora shifters, while on the low end, are sturdy enough, work well, and will last for a few thousand miles. You just want to try and guage how much life is left in them.....
For reference, replacement Sora shifters aren't too bad - about $50 for the right and about $40 for the left:
LEFT
RIGHT
A Tiagra pair runs about $200:
Tiagra set