I want to spend about $100.
Having the unit shut off the computer is less of a concern than having it shut off during a surge or drop.
Scams are easily promoted when claims come without numbers. Let's put some numbers to your problem.
Ideal voltage for all electronics is when incandescent bulbs even dim to 50% intensity. How often are your bulbs dimming that much? How often is your voltage dropping that low?
Low voltage is harmful to motorized appliances. If suffering low voltage, then a refrigerator, air conditioner, dish washer, and washing machine need that low voltage solution. If voltage is not dimming bulbs that much, then power has been perfectly ideal for all electronics.
A voltage reduction is a brownout. Does not harm any electronics. Surges cause high voltage transients. If voltage is so high as to be harmful to electronics, then disconnecting (power off) is too little and much too late. Again, numbers. That existing box (or an APC UPS) takes at least tens of milliseconds to disconnect AC mains or switch to batteries. A destructive surge is done in microseconds. You could have 300 consecutive surges before those boxes did anything. So they conveniently forget to include numbers in the function list. They claim to protect from high voltages that typically are not destructive. And just forget to provide numbers that say so.
As NutBucket and velillen noted, some of the dirtiest power seen in a home comes from a UPS in battery backup mode. Some numbers for this 120 volt UPS that is called a sine wave output. 200 volt square waves with a spike of up to 270 volts. Square waves (as taught in high school mathematics) are nothing more than a sum of pure sine waves. Since they did not make the claim with numbers, then those 200 volts square waves and 270 volt spike are nothing more than sine wave output.
Meanwhile, that 270 volt spike is ideal power for any electronics. Because even 1970 international design standards defined 600 volts spikes without damage. Today’s electronics and design standards are even more robust. As others noted, the crappiest power from a UPS is made irrelevant by circuits already inside every power supply.
Low voltage is never harmful to electronics. Low voltage is harmful to motorized appliances. Transients cannot be stopped. Must be solved elsewhere by a well proven (100+ year old) solution that costs about $1 per protected appliance. And comes with companies with better reputations including Siemens, ABB, General Electric, Intermatic, Leviton, Cutler-Hammer, and Square D.
First identify the anomaly to be solved. For example, how much are your incandescent bulbs dimming. Better solutions are available only after each anomaly is first defined. No magic box exists to solve all. But when making arbitrary (qualitative; not quantitative) claims in a sales brochure, then they can even claim protection from sunspots.
The UPS has only one useful function once we include numbers To provide temporary and 'dirty' power during a blackout. Otherwise connects electronics directly to AC mains where power is 'cleanest'.
First define each anomaly. Different solutions exist for each. For example, how many seconds do your bulbs dimmed to 50% intensity.