I suspect the added option cost will be in the $5-10K range. In all-day direct sun, you might get an average of 2-3 miles of additional range per day. If 80% of the days offer sun, and 80% of the time you park outside all day, oriented towards the sun, that means each year you get 3 * 365 * .8 * .8 = 700 miles of added solar range per year.
The Cybertruck requires about 45 kW per 100 miles or 315 kW for 700 miles. 315 kW * $0.30 = $94/year in savings.
So for $5,000, it will take $5000/$94 = 53 years to break even.
I've provided all the numbers and formulas, so you can play with my values to see your break-even point with different assumptions. I don't see how it makes any sense for Tesla or its customers.
The Solar roof on the Fisker Ocean (a larger solar area than the Cybertruck) in the best case can gain 4 miles of range/day. The Ocean also goes further per kW. So my 3 miles/day estimate seems unlikely in the Cybertruck.