The "correct" way for a non-subscription DDO player to advance in levels without spending too much money is to carefully spread out your limited Turbine points among mid-upper level adventure packs. (In particular, almost all of the leveling content from 10 to the game's level cap of 20 is in paid packs.) Over the weekend, I wandered off of this game plan. It cost me under $2. In the grand scheme of things, that's not actually such a big deal.
My bard was level 5 and had just genuinely struggled to clear the first quest of the Delara's Tomb adventure pack, a workhorse of a story arc that many players ride through to at least level 10. It was pretty clear that I wasn't going to survive part 2 (a level 6 quest) without either gaining some levels or dropping down to "casual" difficulty. So, it was time to go get more experience, only I had already beaten all of the level 4 content I owned, and most of the free level 5 stuff.
Then, I noticed that, with the current sale, the Catacombs adventure pack was currently priced at 175 Turbine Points (approximately $1.75, depending on what exchange rate you happen to be paying). It's one of the cheapest packs in the game, even though the adventure covers a fair amount of content (8 stages by Turbine's count), because all of the content is sitting at level 3, which most players breeze by without needing to pay for premium content. To make a long story short, I decided to spend the points.
I beat the entire pack on normal difficulty in a single sitting - level 5 is the highest level that still gets full exp for a level 3 quest - and gained the last chunk of experience I needed to hit level 6. I may or may not repeat the chain on hard mode (the bonus for hard difficulty a bit more than cancels out the penalty for being over-level). I will certainly level alts, and all of them will now have access to the adventure pack. Even if we imagine that I never touch the thing again, though, that would mean that I spent $2 on an evening of entertainment that happened to help me move towards a goal I was working on in a more entertaining fashion than repeating old dungeons I had already memorized.
$2 in an evening is more than your average MMORPG ($15/month = $0.50 per day), and the content was probably less than 5% of what we'd see in a $40 expansion pack. Even so, the amount is trivial compared to any other form of paid entertainment out there. I typically dislike this type of comparison when we're talking about things that are in an entirely different price class - I don't think that it's valid to compare your MMORPG subscription to expensive sports or theater tickets or cars (yes, I've seen people use that one), because you're not really going to be substituting one for the other - but the numbers in the DDO shop really aren't that far out of the price class, especially if you actually take advantage of continuing non-subscription access to the content you pay for.
I now own the three lowest level adventure packs in the game, none of which are rated on anyone's top 10 highest priority packs. All three were excellent, far better than the free content they compete with. I don't know if that's good for the free to play model (people who buy them will probably be satisfied) or bad (people who don't buy will be stuck with content that is less interesting, which might make them less inclined to stick around). Either way, it's interesting to see how our perception of cost scales.