Well, I am a dentist. Yes, custom made night guards are recommended for those who grind their teeth at night (which is when I would expect most of the damage is being done), not just when you are lifting. The severity of the damage will determine what you can get away with as far as store bought devices. $1,000 sounds excessive…very excessive unless you have worn most of the enamel off of your teeth and this is part of long term treatment.
Most people who grind their teeth aren’t just doing it when lifting. in many weight lifters (including myself), the masseter muscles are more developed and can generate much more force than that seen in a sedentary person. that would lead to more wear at night.
Bottom line, custom devices are recommended in severe cases. No one, not even me, could tell you how severe your case is without looking into your mouth and seeing the damage. that means taking the word of people unrelated to your specific situation about what you should do is a little off. If you are doing significant damage, it will only get worse over time.
As far as the difference between a custom made device and a store bought, I have been trained in this area specifically by the military. The goal is to find a comfortable position when the device is in place in order to help reduce the activity of those muscles, not simply protect the teeth. Using “bite registrations” and setting the head of the condyle of the mandible back into the fossa can help that goal. I often give my patients the soft devices to wear DURING THE DAY in order to help teach new habits if their symptoms imply daily grinding instead of at night.
Considering I do these all day at no cost to military men and women, I find $1,000 bucks for one extreme.
Get a second opinion and ask if a “boil and bite” registration is a possibility.