Here is how I think the economics of battery swapping will work:
The battery swapping system will show that it not only takes less time than Supercharging, but less time than filling up an ICE car with gasoline. This will be attractive to anyone, and especially people for whom time is money. By comparison, charging up the existing battery in your Tesla, while free or nearly free, isn't nearly so cool, as you have to sit and wait.
Therefore it will be acceptable to CHARGE MONEY to swap your battery. The amount of electricity required to charge up a battery is what, $3 ? That would be the minimum acceptable charge for swapping your battery. $5 might be a sensible start. I currently drive a Honda Civic which can cost $40 or more to fill it up, for a 350-mile drive. Paying Tesla $5 will seem like peanuts.
In fact Tesla could charge a lot more than $5... they could even vary the charge according to market conditions - what the market will bear, equivalent price of gasoline...etc.. What if they charged $20 to swap battery, and the price of gasoline was $50/tank?
Tesla owners seem to worry about losing track of "their" battery when they do a swap. As I see it, you will wave bye-bye to your battery forever if you start swapping. You only have a specific battery in your car for a few days, maybe a week, right? Much less if you are driving across the country. You will be using batteries you never saw before and will never see again. To me, it's not a big deal to surrender the initial battery that was in your car "into the system."
Tesla can always say "if you want free fuel forever, just go to the nearest Supercharger. Up from that in cost is charging up at your home. If you want to have am ALMOST-INSTANT recharge, faster than any gasoline car can be filled up, it will cost you money." Tesla could make loads of money.
(to answer the question about different battery pack capacities, the 60kWh pack might cost $5 to swap, the 85kWh might cost $6 to swap and the 100kWh when it comes out might cost $7 to swap. Those ratios would be worked out correctly, and all three prices might wander up and down with the market.)