FWIW, I just returned from a road trip on which I tested my CHAdeMO adapter at four different CHAdeMO stations. One of these (an Electrify America unit) didn't work for me, but the other three did. I got between 33kW and 46kW starting rates, but they did ramp down a bit above about 85% SoC (and got
way low on one, which I charged to 100% SoC just because I was curious about what would happen). I posted about this experience
here, if you care to check it out. Other posts in that thread may be of interest to you, too.
You said that you'd pay "$40 for 10 free charges and $5 per charge after that." In theory, if there's no time or kWh limit on a "free charge," then you could charge from 0% SoC to 100% SoC 10 times, which would give you 750kWh (in an LR; 500kWh in an SR+), for a cost of $0.053/kWh (or $0.08/kWh for the SR+). That's a crazy extreme example, though, and it would take a while to go from 0% to 100% on a CHAdeMO fast charger. The time I charged to 100% on my road trip involved starting at 67% SoC, and that took an hour. Of course, as charge rates taper as you get close to 100%, it likely would not take three hours to go from 0% to 100%.
Let's take some more reasonable figures: Suppose you charge from 30% to 80% in an LR. That's half the battery's capacity, so you'd be consuming 37.5kWh. Based on my recent road trip experience (which might not be reasonable -- see below), 37.5kW is a somewhat conservative guesstimate for the average charge rate, so each such charge would take about an hour. The Model 3 gets something on the order of 250Wh/mi driving efficiency, so that 37.5kWh will take you about 150 miles. Assuming 12,000 miles per year, or 1,000 miles per month, you'd need to charge like this 6.7 times per month, acquiring a total of 250kWh. At $40 per month for the plan, that works out to $0.16/kWh, which I gather is a pretty decent price for Hawaii. This analysis is pretty rough, though, and it does not take into effect the inefficiencies of the DC fast charging process -- on my recent road trip, for instance, my Tesla recorded having received 15kWh at one stop, but the station reported having delivered 16.3kWh. Using that ratio, the price in my scenario goes up to about $0.17/kWh. You'll also need to adjust this based on the amount of driving you do, vs. the 12,000 miles per year I assumed.
One big caveat here is that not all CHAdeMO stations can deliver the maximum of 50kW that the Tesla CHAdeMO adapter can theoretically handle. One of the stations I use delivered 33kW for the entire session. I've heard of stations that only deliver 25kW. If the provider you're considering uses such slow stations, that wouldn't affect the cost computation, but it would require spending more time at the station per charging session. You'll have to decide for yourself how much time you might reasonably want to spend at these locations.
You might also want to look into public Level 2 (J1772) charging facilities. I don't know about Hawaii, but here in Rhode Island, these do exist, and many of them are free (some are not, though). Some are near parks, movie theaters, malls, and other places where people often spend a lot of time, so they can be good ways to charge a car if you don't have good Level 2 charging infrastructure at home -- just charge when you see a movie, hike in a park, or whatever.
For all of these, check
PlugShare for details. Charging speeds don't appear in the charge station descriptions, but review the check-in comments -- some users note their charging speeds.
