> Are L1 vs L2 vs L3 chargers radically different in cost?
Short answer: yes. L1 to L2 isn't that dramatic - it's typically a 240V 40A separate circuit, which is not something to dick around with lightly but pretty easy stuff for a regular electrician. L2 to L3 is a very different ballgame, involving specialist installers and interacting with the power company to get a fatter circuit. Even a single 100kW charger needs a circuit that would be at the high end of what most people get for their whole house, so for a bank of them you're equivalent to a whole subdivision.
I agree with GP that L2 is the way to go for most parking lots. It's enough to provide more charge than the round trip took for most visitors, and you can have a bunch of them. Also you don't have to deal with different charging standards; J1772 is practically universal. You could put in even more L1, but you'll quite likely to have more than you have customers (at one time) to use them and those customers won't get as much charge back as they consumed for the visit. For something like a corporate campus where people stay all day it makes more sense, and an apartment complex likewise except that you'd need some local storage to "time shift" from the sunny day to the occupied night.
Also, while I'm here, I'll raise another point that I haven't seen mentioned. The shade provided by a parking-lot solar array isn't just a matter of comfort or aesthetics. When it comes to charging EVs, it also allows faster charging and/or can help extend battery life. It's rare to find two technologies that complement each other so well.