Can't find network, would it really cost this much to repair

I have an iPhone 6, it has had trouble finding a network (it says can't find network or searching even though I do have service ), some days it can some days it can't. We have tested the SIM card and even though my sim doesn't seem to fit great, I tested my moms sim on mine and it still didn't work. I have tried all trouble shooting and network resets and all that , it's not my carrier either. I took it to this little cell phone repair and at first he said it would most likely cost 50$ to fix , he opened it up and said there wasn't any water damage but the thing that he actually had to fix was almost impossible to and would cost 200-300 dollars , does this sound right? What could it be ? Any other tricks I should try? Should I get second option ?