I gotta be honest, so far growing marijuana has proven to be quite difficult, seedlings need to be cared as babies, but honestly, it wouldn't be expensive/difficult if it was completely legal. Just imagine how much a bunch of seeds would cost? Very cheap, you could just throw a bunch and survival of the fittest but since right now each one is like $14 then that's why I'm caring so much.
Personally I don't see the cost of seeds changing that much if/when cannabis becomes completely legal world wide. The cost of the seed really comes down the strain and the breeder reputation.
I have noticed the cost of some lights/supplies has gone up since more states have gone legal. Maybe it's just inflation, or there's more growers and the demand of supplies causes suppliers to jack up the price because they know someone new to growing will pay it. Example (I have a 150w cfl that I got for 55$, now costs 75$, a DWC kit was once 40$ has now become 60$)
As far as the difficulties we face growing illegal vs legal, aside from legal ramifications, we still face the same issues: Setting up grow rooms, keeping them safe from those who might wish to steal/scam us, dialing in our nutrients, maintaining the climate etc.
Lots of growers have started out knowing very little and by doing some researching online and through forums here, they have been able to produce successful harvests time and time again.
I will agree though, that being illegal has driven the innovations of hiding/protecting the grows. Though thanks to the sneaky bastards trying to get free nugs, those aspects I don't think will change much.