Since the pedal fell off, it's reasonable to assume the OP is talking abouy stripped pedal threads in the arm.
Cost to repair depends on where the OP lives, and the shops in the area. Some are capable of fixing the crank arm with a thread insert or coil, while others will insist on replacing the crank.
To the OP.
1- take a moment now to add your location to your profile so someone might be able to give you a good referral local to you.
2- if there's a bike co-op in town that'll be your best yet. The next best bet would be an old style "bike shop", where they focus on repairs rather than high end new bikes.
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