Free to Play MMOs allow you to spend as much money as you feel like. FtP haters like to point out that this means the amount of money you can spend on a "FtP" MMO is infinite. To have the absolute best gear or have access to the very best content, you might have to pay hundreds of dollars. From this observation, they sometimes go on to make the blanket claim that FtP MMOs are more expensive then sub based MMOs.
In my experience, if you have self control exceeding that of a five year old, FtP MMOs are a usually quite a bit cheaper to play than sub based MMOs. A few example should illustrate my point.
Sub based examples:
Lord of the Rings Online: $50 for the client, $200 for a lifetime sub (would have spent more than $400 on sub fees by now had I not gone lifer). $40 for the Moria expansion, $20 for the mini expansion.
Cost so far: $310.
World of Warcraft: started at launch and subbed for about two months. Box plus one month sub fee, $65. Got back into it again a few months later, played until well after BC. I think I played around two years. BC box $50, two years of sub fees $336 (I would sub in three month blocks). Got sucked in enough buy the WotLK trial to buy that expansion ($50), but didn't last past the free sub month.
Cost so far: $501.
Wizard 101: the client is free. However only about the first 1/10 of the game is accessible for free. You have to pay to unlock later areas as you go. My main is about half way through the game in terms of total content available. However, the design of the game is such that it encourages alts. There are something like 36 possible primary and secondary magic school combinations, and all of them play at least somewhat differently. I have been playing for about two years now, and all I have ever needed is the first three worlds that I have bought access to. Note also that you can simply choose to sub and unlock everything for as long as you stay subbed.
Cost so far: roughly $30.
Dungeons and Dragons Online: the client is free. Sufficient content to get you to level 12 (of 20 levels) or so is free. You pay for additional races, classes, and adventuring zones. I have been playing off and on for over a year and have yet to crack level 8 on any of my characters. All I have purchased so far is access to a new race. Like Wizard 101, you can also choose to sub and unlock everything for as long as you stay subbed if the FtP model really offends you.
Cost so far: less than $20.
Now lets say for the sake of discussion that I was super hardcore and needed to buy access to every single area in the two FtP MMOs to be entertained by them. The last time I checked that would be a grand total of around $100 in Wizard 101. That's lifetime access to the entire game for less than half of what a lifetime subscription costs in a sub based MMO, and a free client.
In DDO, that would not come as cheap. I am too lazy to log on and do the math again, but it was between $200 and $300 to get all the classes, races, and adventuring zones the last time I checked. Not exactly super cheap, but not really any more than you'd expect to pay in a sub based MMO in the long haul. Pluss you get to pick and choose what you actually want. Hate raiding? Don't purchase any of the raid content.
Aha, but what about all those awesome items you can buy in the item shop? I have yet to try any FtP MMO where you can't gear yourself out just fine with items you earn by playing. This is where "having more self control then a five year old" comes into play. More often then not the stuff you can get in ftP MMOs falls into three categories: 1. Fluff/ cosmetic items (ala the pets you can buy in WoW), 2. Convenience items (consumables, Xp boost items, travel items), and 3. Gear with identical stats to what you can earn in game, but that looks cooler.
There are FtP MMOs, where you absolutely need items from a shop to be viable, but in my experience they are the exception to the rule. The one caveat to this is that to all intents and purposes you are usually locked out of the endgame , be it raiding or PvP, if you aren't willing to spend some cash. As I hate raiding, and rarely enjoy PvP in MMOs, that doesn't really affect me. Besides that, I almost never make it to the cap in any MMO in the first place (I can list the exceptions without using up all the fingers on one hand).
It's also a bit of a playstyle issue. If, like me, you are mainly an explorer; FtP MMOs provide great value. You get to explore the mechanics of a new game and see a lot of the content for next to nothing. On the other hand, if you are a hardcore achiever, then I can see where stuff in an item shop would be like crack to you. Spend $5 and instantly become butcher...woot!
Regardless, I don't think that for an average player FtP MMOs are nearly as expensive as a sub based MMOs. Now that the developers of sub based MMOs are starting to blur the line by tacking item shops onto their sub based MMOs (e.g., the cosmetic pets in WoW, the Adventurer's Pack offered by Turbine in LoTRO, and the full on cash shops in Cryptic games) the line between the two business models is starting to get very fuzzy. The quality of FtP MMOs is also converging with that of sub based MMOs rapidly.
If the current trends continue, in a year or two the only difference between a FtP and sub based MMO will be whether you have to pay for the client and server access on top of whatever optional items and content you choose to buy. I know which games I'll be playing if and when we get to that point.