Blown sparkplugs are of two types:
a) someone forgot to properly tighten one and it unscrewed itself (easy to tell as the thread is intact)
b) the piston blew it out, thread and all. Happens mostly on race cars or cars that have been modified by silly persons......
In the case of a) new wire, same plug (if alive), cost zero and some embarassment for SOMEONE!
In the case of b) and have an honest mechanic, they can install a thread insert in about a half an hour and get you going. That won't solve the WHY??? The why lies in either a silly mod (like shaving the head to increase compression ratio without proper measuring of clearances) or using to much boost or "funny fuel".
Hope this helps