Several people here have already got figures, and they've talked about how the figures are a good size and impressive quality. The Cars figures, for instance, are apparently bigger than the diecast Cars line and have a nice amount of detail. If you want to see pictures, check out Sheriff's unboxing threads. Considering that the newest Monsters University action figures at the Disney Store here in the UK are only £2-3 less than a Disney Infinity figure and they're simple toys, not interactive game pieces, I think the current price is pretty fair.
Overall, the price will depend on where you purchase them. Here in the UK, supermarkets are generally cheaper than game stores, so Skylanders figures here might have a pound or two knocked off the recommended retail price. So the price may eventually come down at places like Target and Walmart, although it may be a while until that happens.
The Toy Story playset hasn't yet been officially announced. The Phineas & Ferb playset hasn't yet been announced. We've seen evidence that both exist. Plus it seems extremely likely that Frozen will have a playset. So that's definitely two, possibly three playsets that haven't been announced that we're likely to see this year, and who knows how many may appear next year. Definitely more than the one unannounced playset you mentioned, so that's another thing you don't have to worry about.
There isn't any DLC, but that's what the figures, playsets and power discs are, when you think about it. Instead of paying money to an online gaming network and getting additional content, you pay money for physical objects and they give you additional content (new powers, new worlds/levels, new objects and decorative items).
You won't have to buy the figures again. The figures are compatible with any base, whether it's been bought in a starter pack for the X-Box 360 or the PS3, and that will extend to the X-Box One and the PS4 in the future. The only thing that will be different will be the software disc, which will only work on its corresponding console, same as with any game. If you buy Infinity 2 when it comes out, figures that were released for Infinity 1 will still work with the base and the new software. If you buy figures released for Infinity 2, they'll still work with your Infinity 1 software. You can find out more here -
http://www.disneyinfinityfans.com/viewtopic.php?f=9&t=44I guess the power discs price and its value depends on whether it's something that appeals to you. If you're not interested in changing the scenery and are happy to rely on the scenery provided in the game, or if you're happy with the items you recieve in-game and don't feel the need to expand on that, then there's no pressure to purchase the power discs, so don't worry. They're a bit like Pokemon cards or any other collectable game - if they appeal to you, then $2 would be good value, if they don't then it won't ruin the game experience if you don't have them. They're just a nice perk for those who want them.
You obviously have a lot of concerns about the game, but I hope that eases your worry a little.