I'm sure you thought over the numbers with the team. Not sure if you have to answer to a higher power, such as investors.
I would have opted for Starter Kit with Corona splash screen or positionable watermark upon first load for free, $99/year for the Starter Kit with no splash screen/watermark, $399/year for the Pro, and $999/year for the Enterprise. I so much want Corona to continue to grow in both the SDK and its developer userbase, so giving the Starter Kit for free without having any sort of nod to how that app was made won't help us (meaning Corona), only the developer who might not care for in-app purchases, analytics, or plug-ins.
I came from iTorque 2D and GameSalad. Around the time I stopped using GS, I saw them going "free" as well, with very limited options and their "pro" package was at $299. Their free version doesn't require a splash screen, but - if I may be frank - the entire platform is quite limited overall as to what can be done, compared to Corona. Even with just our Starter Kit, one can create full-blown apps and games that have camera / microphone (working?) access, Game Center support, Android publishing, social networking integration, and full-on internet, local network, and multiplayer capability - whether its Corona Cloud, NoobHub, AutoLAN, PubNub, and HTTP post/get, FTP access, handling RSS feeds, 2D skeletal animation with Spine and Spriter, and many more of which are free or premium Lua modules that people have posted - and I know I'm missing a lot more of the top of my head... but, that's a lot to give away for free without anything in return, and hoping they'll edge up (a big edge up!) to Pro.
Even if Joe Schmo created a fantastic app with the Corona Starter Kit and put it out there for free or $0.99 and it's a hit, you'll have tens of thousands, if not millions, using a known Corona app, and that could pick up additional subscriptions when people realize what they could really do inside Corona. Consequently, I remember the time when people saw the GameSalad splash screen upon launch of a new download, it was immediately trashed by game website blogs and in App Store reviews. That's because the developers using GS couldn't do anything more than "taptard" games. But, such is so not the case with Corona apps. You really can create professional and even native-looking apps and games, and no one might ever suspect it wasn't written in native Objective-C or Java.
Now, that $599/year for Pro might be quite a lean over the mark for Indies who want to dabble in the additional features and have access to plug-ins. I will anticipate that Gluon plugins that Enterprise developers create for the community will more likely be sold rather than given, and I'm not sure if Gluon plug-ins prices will be one-offs or subscription prices in and of themselves. So, with every Gluon plugin they want, they'll already be inching closer to the Enterprise mark ever so quickly. Having the Pro closer to the middle at $399-499/year will make it more palatable. As I stated in previous threads, I have no stomach right now to grasp Objective-C, so I'll probably never bother with buying Enterprise, but you can be sure I'll be getting every worthy Corona Gluon plugin available.
People might not like this particular idea, but I'd even go so far as to break it down and return the option of one platform for free/$99/$249/year for Starter with Splash / Starter / Pro for iOS / OS X (wink wink), free/$199/$349/year for Starter with Splash / Starter / Pro for Android (let's be honest it, it's harder for Corona Labs and us to develop with all of Android's stupid fragmentation), and $499/year for Pro for all platforms.
That's my 2 cents.