What is the proper way to clean/disinfect a very expensive dental night guard other than using more money on otc tablet cleaners?

Brushing: You can brush it gently with water which won't cost you anything. Generic house brand denture tablets cost about $0.08 each and are designed to do what you ask. It's an extremely small price to pay to maintain your dental health investment, and part of the overall cost of your dental rehabilitation which includes implants, crowns, periodontal surgery, and transitional dentures. All the best...

Retainer Brite: is a product made for cleaning removable braces and retainers. It won't harm any metal parts of your niteguard and will keep the plastic parts clean and sanitary. As to the cost, try shopping online.

Toothbrush and soap: The best way to clean your night guard is with a toothbrush and liquid soap like Soft Soap. Do not ever use a mouthwash or alcohol product. Toothpaste can be too abrasive.

Cleaning: Gently brushing with soft toothbrush (no toothpaste) 2x/day will remove surface bacteria. Using denture cleaning tablet 1x/day, as per package instructions, will deactivate the bacteria that impregnate the plastic, that can't be brushed off because they are not on, they are in. Or you can soak in 1part chlorine bleach in 10 parts water for 15 minutes every day. Don't forget to rinse thoroughly.