If I wanted to launch a free-to-play game with different characters, weapons, items and clothes that can be either unlocked through gameplay or bought with real money, what percentage of items should only be available for real money? This isn't "subjective" - I'm interested in examples of real games and estimates of what percentage of their content is free.
The US Virtual Good market (freemium games qualify) is expected to hit 1.6 Billion dollars this year.
0.01% of that is still 100K.
You shouldn't concern yourself with percentage, per se, but with content. Which items are paid for may be more important than how many.
My general rule is this, if an Item can save someone 5-10 minutes during game play, or advance to the next "level" automatically (like a +1 item) charge a buck. Angry Birds uses this idea with the Mighty Eagle.
In all the zynga games I've played, it was either A) advertise for them by friending people, or B) pay money to advance past an hour of play. I don't necessarily like this style, because it frustrates me and forces me to stop playing if I don't have the friends.
In my game (Shining Force style game) I'm making 'promotion' cost money. You can still beat the game with a level 30 fighter, but a level 30 dragoon will be so much cooler.
The problem with asking for a hard figure here, is that this is an extremely variable number.
On the one hand, you have something like World of Warcraft, which, while not exactly Free-to-Play, still might serve as an illustrative example on the one end of the spectrum, with approximately 0% of content that is only available for cash. (The Celestial Steed)
On the other side of the spectrum you have producers like Zynga (of FarmVille fame) who maintain a catalog that is, at a wild guesstimate, about 50-60% cash-only items, and a fair chunk of these items are additionally time-limited (e.g. Halloween stuff, Christmas stuff, etc.) (Note: For various reasons, these numbers are difficult or impossible to pin down to an exact number; but when browsing the store in a game like FarmVille or FrontierVille, it seems as if basically every other item is bought with for-cash currency.)
On the gripping hand, it might be interesting to mention the spectacularly successful game by EA, The Sims 3. If you discount the initial cost of the game, which is relatively low and a one-time expenditure; their model works a lot like a Free-to-Play game.
Another wild guesstimate on my part puts their for-cash catalog at about 50%; unless you count content from the expansions. (A fairly rough breakdown puts cash-only content at about 30% of their catalog; free content at about 30%, and an additional 40% in expansion pack content.) (Note: Again, these numbers are hard to pin down, as they aren't exactly published anywhere; but these rough estimates are based on observation of the EA store for TS3 content.)
And here's the real clever bit what they do: With each expansion they bump the version number, giving players a host of new gameplay features; if you don't buy the expansion, but keep the "free-to-play" version; you only get the framework, and not the content required to make the feature interesting.
How much pay-to-play content you can get away with is strongly related to how much the game costs to play. In a Free-to-play game, you can probably get away with as much as 60-70%; but a more optimal popularity/profit-scale is probably achieved from setting it at about 50%.
Additionally, as demonstrated by the WoW example above; the ratio is not as important as the price and relative coolness of the item. Even if you only have one cash-only item, if it's insanely awesome you can make an incredible amount of money from it, even if it's just cosmetic, so long as you have enough players. (Estimates put the Celestial Steed at about $2.5 million of income, at the very least.)
Summary of the summary
I've recently locked most of my game; despite having really few users you might be interested.
There are 6 classes ("levels"), and non-paying users may only acquire class 0 or class 1 players and weapons. Also, they can't do direct trading with other users, but they are allowed to use the auction (still can buy only class 0 or class 1 stuff).
Moreover, I have set up an exclusive conference, that will be the "main" conference, only for paying users. Non-paying users can still play as much as they want in the "lesser" conference.
Well you'ed have to guess on it really since most of the games change there what the player can only buy with real money list, sorry best thing I could think of. Most of the games I've played with free-to-play usally keep around, and I'm guessing a bit, like 5-10 percent like Joe Wreschnig said, but it is really up to how big it is. If it is like a small game, like im guessing yours would be at the beggining, they make almost everything avaible in-game but really hard to get like a very rare drop and might have a few items that you can only buy, but it gives you a big advantage.
Check games like holy-war, Shogon Kingdom, and maybe game company's like perfect world if you want to reasearch your self.