*** -About the price** : Riot would have to determine. You have to figure how much money they make of off smurf accounts. How many "smurfs" do you see that buy all of their mains and face roll early on. So on a strictly business stand point, it would be completely up to Riot to ensure they aren't making it "cheaper" to just buy a loaded account.
*** -As far as the "any low level could buy one and ruin higher level games" **: Perhaps make it to where it has to be linked to your main account. Put a button in the store to "purchase smurf" that unlocks once you've played to lvl 30. That way people who aren't already 30 on another account can't just buy one for whatever.
*** - Options?** : I like the idea you gave for "unlocking all the 450-1350 champs" but I feel like that includes a lot of champs not everybody plays. And typically, if you're smurfing, you just want your mains.
I think just selling a plain Jane smurf account, with no champs unlocked would be an easy way to make it a little cheaper. You're either good enough to deal with the free rotation, or plan on buying your main anyway - but this last one is just an opinion.
Not to mention, the 450-1350 IP champs...that's a really long list of things RIot would just be giving away for free...33 to be exact

When you figure the price of all of them together in RP is 15,405...
you can buy 15,000 RP for $100 but you'd still have to spend $5 to get that last 405 (the 650 option)
Riot isn't and CAN'T give away a full account, plus $105.00 to each player.
- **Encouragement? **: I think it'd do a fine job of keeping some of the "I just want to try ranked again" players out of the low levels. I have a smurf account myself, that I made just to level with a friend who just started playing so we could queue together - and the amount of cocky A holes that are duo / triple queuing together just to roll up to 30 is atrocious, and don't teach the new players anything about the game, balance, or the community. It looks bad.